The answer may be found in the Brahmanas. Here is Satapatha Brahamana verse which alludes answers to your questions:
SB 6:1:2:11
And so they say, 'Pragâpati, having created these worlds, was firmly
established on the earth. For him these plants were ripened 2 into
food: that he ate. He became pregnant. From the upper vital airs he
created the gods, and from the lower vital airs the mortal creatures.'
In whatever way he created thereafter, so he created; but indeed it
was Pragâpati who created everything here, whatsoever exists.
SB 6:1:2:36
As to this they say, 'For what object is this fire (altar) built?
Having become a bird, he (Agni) shall bear me to the sky!' so say
some; but let him not think so; for by assuming that form, the vital
airs became Pragâpati 1; by assuming that form, Pragâpati created the
gods 2; by assuming that form, the gods became immortal and what
thereby the vital airs, and Pragâpati, and the gods became, that
indeed he (the Sacrificer) thereby becomes
SB 3:1:1:1
They choose a place of worship. Let them choose (the place) which lies
highest, and above which no other part of the ground rises 1; for it
was from thence that the gods ascended to heaven, and he who is
consecrated indeed ascends to the gods. He thus sacrifices on a place
of worship frequented by the gods; but were any other part of the
ground to rise above it, he would indeed be lowered while sacrificing:
let them therefore choose (the place) which lies highest
As per SB, after having created heaven , earth, sky, plants, men and animals, Prajapti and Gods started to perform sacrifice. Men, animal, and plants become a mean of sacrifice out of which man was the first victim of sacrifice
SB 6:2:1:2
He saw those five animals,--the Purusha (man), the horse, the bull,
the ram, and the he-goat. Inasmuch as he saw (pas) them, they are
(called) cattle (pasu)
SB 6:2:1:18
A man (purusha) he slaughters first, for man is the first of animals;
then a horse, for the horse comes after man; then a bull, for the bull
(or cow) comes after the horse; then a ram, for the sheep comes after
the cow; then a he-goat, for the goat comes after the sheep: thus he
slaughters them according to their form, according to their
excellence.
And since gods knew highest point to perform sacrifice and hence they went to heaven which men do not know yet
SB 3:1:1:1
They choose a place of worship. Let them choose (the place) which lies
highest, and above which no other part of the ground rises 1; for it
was from thence that the gods ascended to heaven, and he who is
consecrated indeed ascends to the gods. He thus sacrifices on a place
of worship frequented by the gods; but were any other part of the
ground to rise above it, he would indeed be lowered while sacrificing:
let them therefore choose (the place) which lies highest
