As mentioned in What are the different names of Brahma as per scriptures? post, Lord Brahma is known as Hiranyagarbha, Aja and Svayambhu:

• Because the word garbha means womb and since Brahma originated from inside a golden (hiranya) egg, he is known as Hiranyagarbha.
• He had no birth (janana) in the real sense of the term. He is therefore also referred to as Aja (without birth).
• It is also true that he created (bhuva) himself (svayam), It is because of this that Brahma is known as Svayambhu.

So, in the very beginning Brahma originated from the cosmic egg.

As per chapter 9 of Kurma Purana,

At the end of a Kalpa (which is just a day of Brahma), lord Vishnu entered the body of Brahma and

22. He became surprised on seeing this entire set of the three worlds, including Devas, Asuras and human beings in the abdomen of that god ( Brahma ) .

23. Thereafter Vishnu, the deity with the Lord of serpents, Sesha as his abode, came out of his mouth and spoke to Pitamaha (god Brahma, the grand-sire of the world).

24. O excellent Purusha, in this manner you too enter my eternal abdomen and see all these wonderful worlds.

But when, Lord Brahma entered in Lord Vishnu's abdomen, Lord Vishnu closed all openings. Later, Lord Brahma found a lotus emerging from Vishnu's navel and came out from this lotus and asked Lord Vishnu why he closed all the openings.

[Lord Vishnu to Brahma:]

34. It was only for fun. It was not with a desire to harass you. Who would wish to trouble the grandfather, the Lord of Devas?

35. O Brahma, you are not harassed. In every respect you are worthy of being honoured. O auspicious one, please forgive me the offence committed against you (i.e. if I have offended you). [v.l. whatever has been done by me to you is int he interest of your progeny.]

36. O Brahma, for this very same reason be a son unto me. O deity in the form of the universe, be famous by the name Padmayoni (having lotus as the source of origin) for the sake of my love.

37. Then that Lord [Brahma] conferred the boon to the crowned one (i.e. Vishnu).

The same chapter explains that Brahma also asked a boon from Shiva to born as his son. And later on Shiva was born as Brahma's son Rudra.

It's out of a boon that Hiranyagarbha Brahma appeared as Vishnu's son i.e Padmayoni Brahma. This is same as how Shiva appeared as Brahma's son i.e. Rudra. Due to these boons, it's believed that Vishnu's son is Brahma and Brahma's son is Rudra.

But as explained in What is the Life span of the Hindu Gods.? post, Brahma, and even Vasudeva and Sankara are created by Brahman (the Ultimate Reality). He alone devours them again, and their remanifestation also take place. Brahman (the Ultimate Reality) can be related with AdiNarayana, Mahākāla, SadaShiva and AdiShakti as per different sects' belief.