There’s very little agreement among scholars as to how to answer that. One attempt, by The Jesus Seminar, came up with some sayings they believed really came from Jesus, and some deeds, but even their conclusions are subjective, and their aggregate report paves over the fact that there wasn’t in fact agreement on those things (every one likely had some dissenting voice; all they did was zero in on the few things that had the widest agreement).

Much obliged, amigo. May you long continue wielding the sharp sword of TRVTH.

Paul might deny it of course (that’s what he’s doing in Galatians 1), but almost certainly Paul learned the details from them through some channel or other (probably not directly; most likely from interrogating Christians he was persecuting). The cosmic model would certainly have been crafted (consciously or unconsciously) by Cephas (Peter) with perhaps influence from the other early leaders of whatever sect they were before this “revelation” sent them end-times-declaring.

How they did that is by pesher, a common practice in counter-cultural Jewish sects when Christianity began and a technique the first Christians were clearly using. We already see cosmic messianism being read out of scripture in the Qumran pesherim. Those documents may even come from the sect that later spawned Christianity. See OHJ, Ch. 4, Elements 5 through 9, and then 16 through 18; and with context in Ch. 5, Elements 23 through 29.

Now, where would the Holy Ghost get David’s seed from, if not from some “cosmic sperm bank”? They might not like that term. And they may argue that the insemination could not possibly have happened anywhere but on Earth. I suppose they might think the Holy Ghost kept David’s sperm on Earth all that time, like Zoroaster’s sperm was supposed to be stored in a lake somewhere. But it just seems everyone is just freaking out on this issue rather than even trying to make some argument.