Hiranyagarbha is the home of all creation, Brahman , Nirguna Brahman and Sarguna Brahman are all within and without Hiranyagarbha. Is Hiranyagarbha the home of Satcitananda?
You are not the first to be confused by this. This is clarified in several Brahma Sutra verses (all available here - https://www.wisdomlib.org/hinduism/book/brahma-sutras). The first verse to discuss this is verse 1.3.13 (Adhikarana 4). It says:
Adhikarana summary: The Highest Person to be meditated upon is the Highest Brahman
In the last section the word ‘Akshara’, though it generally means syllable, was interpreted to refer to Brahman on account of the characteristic quality of supporting everything and we had to go to the etymological meaning of the word Akshara viz. that which does not perish or undergo change i.e. Brahman. Similarly in the text to be taken up for discussion the opponent holds that on account of the attainment of Brahmaloka as the result of the meditation we have to take by the Highest Person the Lower Brahman or Hiranyagarbha which is relatively speaking higher, and not the Higher Brahman.
13. Because of his being mentioned as an object of (the act of) seeing, he (who is to be meditated upon is Brahman).
“Again he who meditates with the syllable ‘Om’ of three Matras (A-u-m), on the Highest Person” etc. (Pr. 5. 5).
A doubt arises whether the Highest Brahman or the Lower Brahman is meant, because, in 5. 2 both are mentioned, and also because Brahmaloka is described as the fruit by the worship of this Highest Person. The Sutra says that this Highest Person is the Highest Brahman and not Hiranya-garbha (the Lower Brahman). Why? Because the paragraph ends thus : “He sees the Highest Person” which shows that he realizes or actually gets identified with the Highest Person. It is not a mere imagination but an actuality, for the object of an act of seeing is an actuality, as we find from experience. But Hiranyagarbha is an imaginary being, since it is a product of ignorance. Hence the Highest Person means the Highest Brahman, which is a reality, and this very Brahman is taught at the beginning of the paragraph as the object of meditation, for it is not possible to realize one entity by meditating on another.
The attainment of Brahmaloka by the worshipper should not be regarded as an insignificant fruit of the worship of the Highest Person, for it is a step in gradual emancipation (Krama Mukti). First he attains this Loka and then final beatitude.
Further on, in verse 2.4.2 (Adhikarana 1):
2. On account of the impossibility (of explaining the origination in a) secondary sense.
Since there are texts like the one quoted from the Sat. Br. which speak of the existence of the organs before creation, why not explain the texts which describe their creation m a secondary sense ? This Sutra refutes it, for a secondary sense would lead to the abandonment of the general assertion, “By the knowledge of one, everything else is known.” Therefore they are produced from Brahman. The reference to the existence of the Pranas (organs) before creation in Sat. Br. is concerning Hiranyagarbha, which is not resolved in the partial dissolution of the world, though all other effects are resolved. Even Hiranyagarbha is resolved, however, in complete dissolution (Mahapralaya).
and further in verses 3.3.16-17 (Adhikarana 7):
16. (In the Aitareva Upanishad 1.1) the Supreme Self is meant, as in other texts (dealing with creation), on account of the subsequent qualification.
“Verily in the beginning all this was the Self, one only; there was nothing else whatsoever” etc. (Ait. 1. 1.). Does the word ‘Self’ here refer to the Supreme Self or to Hiranyagarbha? It refers to the Supreme Self, even as the word ‘Self’ in other texts dealing with creation refers to It and not to Hiranyagarbha: “From the Self sprang forth ether” (Taitt. 2. 1). Why? Because in the subsequent text of the Aiteraya we have, “It thought, ‘Shall I send forth worlds?’ It sent forth these worlds” (Ait. 1. 1-2). This qualification, viz. that ‘It thought’ before creation, is applied to Brahman in the primary sense in other Sruti texts. So from this we learn that the Self refers to the Supreme Self and not to Hiranyagarbha.
17. If it be said that because of the context (the Supreme Self is not meant, but Hiranyagarbha), (we reply that) it is so (i.e. the Supreme Self is meant) on account of the definite statement (that the Atman alone existed at the beginning).
In the Aitareya Upanishad 1. 1 the Self is said to have created the four worlds. But in the Taittiriya and other texts the Self creates ether, water, etc.— the five elements. Now it is well known that creation of the worlds is by Hiranyagarbha with the help of the elements created by the Supreme Self. So the Self in the Aitareya cannot mean the Supreme Self but Hiranyagarbha. The Sutra refutes it and says that on account of the statement, “Verily in the beginning all this was the Self, one only” (Ait. 1. 1), which declares that there was one only without a second, it can only refer to the Supreme Self and not to Hiranyagarbha. Therefore we have to take that the Supreme Self after creating elements as described in other Sakhas created the four worlds.
The object of Sutras 16 and 17 in establishing that the Supreme Self is meant is that the attributes of the Supreme Self given in other places are to be combined in the Aitareyaka meditation.
Hiranyagarbha, or Brahma, there are several names for the Saguna Brahman in It's creative aspect as the creator of living beings. But it is not to be confused with the Highest, Nirguna Brahman.
Hiranyagarbha is actually Saguna Brahman in action. He is not Nirguna Brahman or the Highest cause. He is in fact the effect of the cause.
So, it is not right to say that "Saguna and Nirguna Brahman are within Hiranyagarbha".
Here is the relative status of Hiranyagarbha when compared to Purusha, Avyakta, Manas etc.
Indriyebhyo parA hyarthA arthebhyashcha param manah |
Manasantu parA buddhirbuddherAtmA mahAn parah ||
Mahatah paramavyaktamvyaktAt purshuh parah |
PurushAnna param kinchit sA kashtha sA parA gatih ||
Artha (or the objects) is superior to the senses (Indriya). Superior to the objects is the Mind (Manah). Suprerior to Mind is Buddhi (intellect) but superior to Buddhi is Hiranayagarbha (mentioned here as MahAn AtmA). Superior to Hiranayagrabha again is avyakta and superior to avyakta is Purusha. And there is nothing superior to Purusha. It is the ultimate destination.
So, by equating Purusha to the supreme, you can now get an idea about the position of Hiranyagarbha.
And, in the ShwetAshwataropansishad (Swe U), Lord Rudra is praised as the supreme Purusha. One verse there says that at beginning of creation, Rudra gave birth to Hiranyagarbha.
Unlike the previous verses, here, the word Hiranyagarbha is explicitly mentioned.
Yo devAnAm prabhavashchodbhavashcha vishwAdhipo rudro maharshih |
Hiranyagarbham janayAmAs purvam sa no budhyA shubhayA samyunaktu ||
Let the all-knowing Rudra, the Lord of the universe (viswAdhipo), who is source of creation of all deities and the reason for their prosperity, and who created (or gave birth to) Hiranyagarbha at the onset of creation, bestow us with auspicious intellect.
Swe U 3.4
So, the highest cause manifests as Hiranygrabha (the effect) at the beginning of creation and pervades the creation.
NOTE - The Upanishad books that I have are having explanations/commentary as given by Anandagiri, Adi Shankara and the translation is as per SwAmi GambhirAnanda.