> e.g., where do I put crate::, vs module X, vs module X{ } again? Do I have to specify it in the automatic lib.rs too?

Sure, but there's one clear answer to that question which you can refresh your memory of in under 3 minutes of reading.

C does live in the filesystem, but where in the filesystem? Oh and it doesn't always live in the filesystem--sometimes it lives in the package system.