“If you have an eternal body, then why are you known to be born from Brahma in this Svayambhuva-manvantara? It is said utsangan narado jajne: Narada was born from the deliberation of Brahma, which is the best part of the body. (SB 3.12.23)”

That is true. Just as the Lord for his particular pastimes enters the womb of Devaki, from the pastimes of being the son of Brahma, at the end of the previous kalpa (Brahma’s previous day), I entered the body of Brahma. When Brahma withdrew the three worlds (idam adaya), I entered along with his breathing into Brahma who desired to go to sleep in Narayana (sayane), who was sleeping in the water of the only ocean (udantavah).

tato ’vatirya visvatma deham avisya cakrinah |
avapa vaisnavim nidram ekibhuyatha visnuna ||

Having appeared, Brahma, entering the body of Visnu, becoming one with Visnu, then went into Visnu’s spiritual sleep. Kurma Purana

Another version has svayane instead of sayane. This means “in his controller, who is the water.” The expression equates Narayana with the water, because he is non-different from it.