I think fanfiction starts that way, but a lot of worm fiction is written by people who did not read or did not like the source material. They read a few stories written by fanfiction authors and go from there. Eventually you get ideas from the fan base that takes on a life of it’s own. This is especially true of minor characters who get fleshed out really well in some specific work.

It’s like how Superman’s source material is self contradictory where individual works are different. So, you can have Smallville fanfiction or more generic Superman mythology fanfiction.