The unmanifest is He who is beyond the range of sense perception and so on. Who is that? That will be described in the Ayodhyā-kāṇḍa of Rāmāyaṇa (2.110.5) thus:

ākāśa-prabhavo brahmā śāśvato nitya avyayaḥ
tasmān marīciḥ saṁjajñe marīceḥ kaśyapaḥ sutaḥ

“From the self-effulgent Lord was born Brahmā who is eternal, ever-lasting and inexhaustible. From him, Marīci was born. Marīci’s son was Kaśyapa.”

And who is that self-effulgent Lord [who generated Brahmā]? That will be clear in the Uttara-kāṇḍa of Rāmāyaṇa (7.104.4, 7):

saṅkṣipya hi purā lokān māyayā svayam eva hi
mahārṇave śayāno ‘psu māṁ tvaṁ pūrvam ajījanaḥ

[Lord Brahmā spoke to Lord Śrī Rāmacandra:] “Having wound up the worlds in the past by Your illusory potency, while sleeping on the great ocean, You created me in the past.”

padme divye ’rka-saṅkāśe nābhyām utpādya mām api
prājāpatyaṁ tvayā karma sarvaṁ mayi niveśitam

“Having generated me on a divine lotus shining like the sun, You entrusted me with the entire responsibility of a Prajāpati.”

Śāśvataḥ (“eternal”) indicates that Lord Brahmā lives for a long time. Nityaḥ (“ever-lasting”) indicates that he is not destroyed during the two parārdhas [the duration of a brahmāṇḍa]. Avyayaḥ (“inexhaustible”) indicates that he remains in every kalpa as he is, like a stream [of water in a waterfall].

NOTE. The Supreme Personality of Godhead has presented a genealogical synopsis of the universal population in the Bhagavad-gītā (10.6) thus:

maharṣayaḥ sapta pūrve catvāro manavas tathā
mad-bhāvā mānasā jātā yeṣāṁ loka imāḥ prajāḥ

“The seven great sages and before them the four other great sages and the Manus [progenitors of mankind] come from Me, born from My mind, and all the living beings populating the various planets descend from them.”