Soma is not a metaphor rather it represents two distinct yet related things - ONE is the Moon-god also known as Chandrama and other is the invigorating drink that was consumed by the gods. They are both inter-related hence there is sometimes confusion regarding its real nature but the following excerpts should be able to help us understand the distinction as well as their relatedness.
The Shatpatha Brahmana 1.6.3 mentions:
- The sun, indeed, relates to Agni, and the moon to Soma; the day relates to Agni, and the night to Soma; the waxing half-moon relates to Agni, and the waning one to Soma.
The Section 1.6.4 details this further through the following verses:
- The gods said, 'Nothing but Soma will satiate him: let us prepare Soma for him!' They prepared Soma for him. Now this king Soma, the food of the gods, is no other than the moon.
The verse ahead actually clarifies how Soma can be both the moon as well as the food of gods:
When he (the moon, masc.) is not seen that night either in the east or in the west, then he visits this world; and here he enters into the waters and plants. He is indeed a treasure for the gods, he is their food. And since during that night he here dwells together (amâ vas), therefore that (night of new moon) is called amâvâsyâ (the dwelling together, or at home).
They prepared it (Soma for Indra), after having it collected, part by part, by the cows: in eating plants (they collected it) from the plants, and in drinking water (they collected it) from the waters. Having prepared and coagulated it, and made it strong (pungent), they gave it to him.
In the same way as the Soma stalk becomes strong (by being touched or sprinkled with water), so he (Indra) became strong (by the Soma being mixed with boiled milk) and overcame that evil, the jaundice. Such is likewise the significance of the new-moon ceremony (and the Sânnâyya, or libation of sweet and sour milk offered to Indra thereat); and verily he who, knowing this, mixes (sweet and sour milk at the new-moon sacrifice) in like manner increases in offspring and cattle, and overcomes evil: let him therefore mix together (sweet and sour milk)
This means Soma, the moon-god comes and resides in the water bodies and plants on Amavasya night and his essence is collected through the plants with water. How do cows come into the picture? Because they consume plants and water and produce milk so their milk is added to the mixture but only when it is collected on new moon days.
But as they (the cows), previously (to the new moon), eat mere plants (not imbued with the moon or Soma), and drink mere water, and yield mere milk,--so that (milk which they offer on the day before new moon, is not imbued with Soma, is ordinary milk). For king Soma, the food of the gods, indeed, is no other than the moon. When he is not seen that night either in the east or in the west, then he visits this world, and here enters into the waters and plants. Having then collected him from the water and plants, he (the performer of the Sânnâyya) causes him to be reproduced from out of the libations; and he (Soma, the moon), being reproduced from the libations, becomes visible in the western sky.
Thus during that night (of new moon) food moves away from the gods and comes to this world. Now the gods were desirous as to how that (food) might (be made to) come back to them; how it might not perish away from them. For this they put their trust in those who prepare the libation of sweet and sour milk (sânnâyya), thinking, 'when they have prepared it, they will offer it to us.' And, verily, in him, who knows this, both his own kin and strangers put their trust; for in him, who attains to the highest rank, people indeed put their trust.
That is the reason oblations of Soma are given to the gods to offer them the food that was theirs and had come to earth! These verses also tell us that Soma was not just plant juice but was a mixture that also used cow-milk.
Coming to the plant we find the entire Mandal 9 of the Rig Veda dedicated to Soma. It has some really interesting verses that tell us how it was derived and distilled from the plants and am sharing the very first one here:
1 In sweetest and most gladdening stream
flow pure, O Soma, on thy way,
Pressed out for Indra, for his drink.
2 Fiend-queller, Friend of all men, he hath with the wood attained unto
His place, his iron-fashioned home.
3 Be thou best Vṛtra-slayer, best granter of bliss, most liberal:
Promote our wealthy princes' gifts.
4 Flow onward with thy juice unto the banquet of the Mighty Gods:
Flow hither for our strength and fame.
5 O Indu, we draw nigh to thee, with this one object day by day:
To thee alone our prayers are said
6 By means of this eternal fleece may Sūrya's Daughter purify
**Thy Soma that is foaming forth.
7 Ten sister maids of slender form seize him within the press and hold
Him firmly on the final day.**
8 The virgins send him forth: they blow the the skin musician-like and fuse
The triple foe-repelling meath.
9 Inviolable milch-kine round about him blend for Indra's drink,
The fresh young Soma with their milk.
10 In the wild raptures of this draught, Indra slays all the Vṛtras: he,
The Hero, pours his wealth on us.
The verse above indicate that the juice was 'pressed' out of the plant and mixed with milk. Not sure what the ten maidens refer to but it could be a wooden strainer similat to the ones used by Parsis. They use a mortar (hawan) and pestle (dastag/abar-hawan/labo) for pounding and extracting the juice from the plant, and a nine-holed strainer (surakhdar tasjta), and a bowl for holding the juice.
The next Rig Veda Book 9 Hymn 2 also helps us understand this further by mentioning a purifying cloth which probably acted as a filter fro the draught:
1 Soma, flow on, inviting Gods, speed to the purifying cloth:
Pass into Indra, as a Bull.
2 As mighty food speed hitherward, Indu, as a most splendid Steer:
Sit in thy place as one with strength.
3 The well-loved meath was made to flow, the stream of the creative juice
ne Sage drew waters to himself.
4 The mighty waters, yea, the floods accompany thee Mighty One,
When thou wilt clothe thee with the milk.
5 The lake is brightened in the floods. ?Soma, our Friend, heaven's prop and stay,
Falls on the purifying cloth.
6 The tawny Bull hath bellowed, fair as mighty Mitra to behold:
He shines together with the Sun.
7 Songs, Indu, active in their might are beautified for thee, wherewith
Thou deckest thee for our delight.
8 To thee who givest ample room we pray, to win the joyous draught:
Great are the praise& due to thee.
9 Indu as, Indra's Friend, on us pour with a stream of sweetness, like
Parjanya sender of the rain.
10 Winner of kine, Indu, art thou, winner of heroes, steeds, and strength
Primeval Soul of sacrifice.
The last verse also reaffirms that it is the Primeval soul of sacrifice re-affirming the story from the Shatpatha Brahmana. The very next hymn mentions that the God Soma, (through the plant juice) made ready with the hymns, was stored in vats of wood:
1 HERE present this Immortal God flies, like a bird upon her wings,
To settle in the vats of wood.
2 This God, made ready with the hymn, runs swiftly through the winding ways,
Inviolable as he flows.
3 This God while flowing is adorned, like a bay steed for war, by men
Devout and skilled in holy songs.
4 He, like a warrior going forth with heroes, as he flows along
Is fain to win all precious boons.
5 This God, as he is flowing on, speeds like a car and gives his gifts:
He lets his voice be heard of all
6 Praised by the sacred bards, this God dives into waters, and bestows
Rich gifts upon the worshipper.
7 Away he rushes with his stream, across the regions, into heaven,
And roars as he is flowing on.
8 While flowing, meet for sacrifice, he hath gone up to heaven across
The regions, irresistible.
9 After the 'way of ancient time, this God, pressed out for Deities,
Flows tawny to the straining-cloth.
Rig Veda Book 9 Hymn 6 re-affirms our notion that the ten maidens mentioned in the first hymn could be referring to the sieve:
1 SOMA, flow on with pleasant stream, a Bull devoted to the Gods,
Our Friend, unto the woodden sieve.
2 Pour hitherward, as Indra's Self, Indu, that gladdening stream of thine,
And send us coursers full of strength.
3 Flow to the filter hitherward, pouring that ancient gladdening juice,
Streaming forth power and high renown.
4 Hither the sparkling drops have flowed, like waters down a steep descent
They have reached Indra purified.
5 Whom, having passed the filter, ten dames cleanse, as ’twere a vigorous steed,
While he disports him in the wood,—
6 The steer-strong juice with milk pour forth, for feast and service of the Gods,
To him who bears away the draught.
7 Effused, the God flows onward with his stream to Indra, to the God,
So that his milk may strengthen him.
8 Soul of the sacrifice, the juice effused flows quickly on: he keeps
His ancient wisdom of a Sage.
9 So pouring forth, as Indra's Friend, strong drink, best Gladdener! for the feast,
Thou, even in secret, storest hymns.
10 This Lord of many Holy Laws, even at his birth engendering strength,
Effused, flows onward in a stream.
In the Hymn 7 we are told that the filtered juice is golden in color:
5 When purified, he sits as King above the hosts, among his folk,
What time the sages bring him nigh.
6 Dear, golden-coloured, in the fleece he sinks and settles in the wood:
The Singer shows his zeal in hymns.
7 He goes to Indra, Vāyu, to the Aśvins, as his custom is,
With gladdening juice which gives them joy.
The next hymn RV Book 9 Hymn 8 also mentions adding milk to the juice to make it fit for the gods:
1 OBEYING Indra's dear desire these Soma juices have flowed forth,
Increasing his heroic might.
2 Laid in the bowl, pure-flowing on to Vāyu and the Aśvins, may
These give us great heroic strength.
3 Soma, as thou art purified, incite to bounty Indra's heart,
To sit in place of sacrifice.
4 The ten swift fingers deck thee forth, seven ministers impel thee on:
The sages have rejoiced in thee.
5 When through the filter thou art poured, we clothe thee with a robe of milk
To be a gladdening draught for Gods.
6 When purified within the jars, Soma, bright red and golden-hued,
Hath clothed him with a robe of milk.
7 Flow on to us and make us rich. Drive all our enemies away.
O Indu, flow into thy Friend.
8 Send down the rain from heaven, a stream of opulence from earth. Give us,
O Soma, victory in war.
9 May we obtain thee, Indra's drink, who viewest men and findest light,
Gain thee, and progeny and food.
There are many other verses that I can share but the ones above should suffice to say that Soma is a drink that was made by combining juice from a particular plant and mixed with cow-milk collected on the new moon day. The verses that talk about him stationed in the heavens and such are most probably referring to the Moon-god who comes down on new-moon nights and lives on earth as evident from the next hymn:
1 THE Sage of Heaven whose heart is wise, when laid between both hands and pressed,
Sends us delightful powers of life.
2 On, onward to a glorious home; dear to the people void of guile,
With excellent enjoyment, flow.
3 He, the bright Son, when born illumed his Parents who had sprung to life,
Great Son great Strengtheners of Law.
4 Urged by the seven devotions he hath stirred the guileless rivers which
Have magnified the Single Eye.
5 These helped to might the Youthful One, high over all, invincible,
Even Indu, Indra! in thy law.
6 The immortal Courser, good to draw, looks down upon the Seven: the fount
Hath satisfied the Goddesses
7 Aid us in holy rites, O Man: O Pavamana, drive away
Dark shades that must be met in fight.
8 Make the paths ready for a hymn newer and newer evermore:
Make the lights shine as erst they shone.
9 Give, Pavamana, high renown, give kine and steeds and hero sons:
Win for us wisdom, win the light.
Regarding your other query, yes it was very much available on earth as shown by the verses I shared from Shatpatha Brahmana. It was available to humans as well but was probably used mainly for oblations to the gods. The method of preparation or the recipe can perhaps be gleaned from the numerous verses that are dedicated to it but the identification of the plant is what is missing right now.