Paul visited Jerusalem/Peter to know it.
• …[It's plausible: Paul received tons of Jesus biography from the Jerusalem church](). [Forthcoming]
• …[In Gal 1:18, Paul says he stayed at Peter's “to get information”]()
Paul had clear reason to want to know Jesus biography.
• …Paul saw Jesus as incredibly interesting/valuable (his hero). [Forthcoming]1
• …Paul aimed to have influence over people/churches. [Forthcoming]
• ……Paul liked to make his points by alluding to Jesus’s life. [Forthcoming]
• ……Paul liked to base his teachings on Jesus’s teachings. [Forthcoming]
• ……Paul wanted be educated (and to come off as educated). [Forthcoming]
• …Paul desired to bequeth self-defining Jesus traditions on new churches. [Forthcoming]2
James Dunn: “And we can also speak without exaggeration of a universal curiosity regarding the prominent or hero figure, which is as evident in ancient writings as it is today” (Note, e.g., the degree of biographical interest evident in Dio Chrysostom in the lite and teaching of Diogenes, or, on the Jewish side, in Jeremiah preserved (by his disciples) in "The Words of Jeremiah" (Jer. I.I. i.e., canonical Jeremiah).)
[See this argument and response here]