So if Mark (believed to be the oldest Gospel IIRC) was like 20 when Jesus died, and then ~100 years later his story was written down. He was writing it down when he was 120? Seems more likely that Mark's 3rd generation students wrote it down after they realized that nobody had even second hand accounts of what had happened. Right? Mark had a mixture of first hand (things he saw) and second hand (things he heard about from people that saw them) accounts, his direct pupils had second & third hand accounts (things Mark told them, and they learned through correspondence with other Christian witnesses), but after 100 years those guys are dying too.

How do they compile all the teachings they've received? How much of what they were taught was Mark and how much was Jesus?

It seems like not a good way to get the message out....

Jesus died so that whosoever believeth in him shall not die but shall have ever lasting life... but don't worry about like writing any of this down or anything for like 100 years. What does it mean to believe in him? I think that's clear enough, we don't need to elaborate.