>You don't write README.md, ARCHITECTURE.md, JavaDocs, etc, at all?

Maybe? Not sure - haven't worked on enough big projects to declare answer

>A new developer is supposed to go through all the codebases and figure out for himself?

Yes to this bit - code should have very clear structure to it with appropriate comments attached, whether that's literate style as JonesForth<https://github.com/phf/forth/blob/master/x86/jonesforth.S> shows, small section header comment or whatever.

It should be very clear what section exploring does - tiny example but cat<http://9p.io/sources/plan9/sys/src/cmd/cat.c> from plan 9 very simple segmented code - deals with file opening in main loop and cat function just reads and prints text