John was written much later. This is not discussing him. Just the synoptic authors, Matthew, Mark, Luke.
“John was written much later.”
Except that the internal evidence is that it was written very early (see the Tresmontant book); although perhaps revised later at the end of John’s life and released in Greek.
Thiede claims that a fragment of Mark dated to 50 AD was found at Qumran (there is a lively debate over that).
But all the Christians at the start were united, in Jerusalem; their story was therefore single; what they believed was shared among them; and it was very possible for them to take that unified single message out to the world—even basing it on common written records which were theirs.