That almost no-one uses directly, but pretty much everyone you mentioned definitely pulls from it(or pull from things branched from it). It would be super interesting to see a path map of commits and what trees they came from for the big distributions of the kernel(i.e. android, debian, fedora, etc). But I'd bet that all of them pull from Linus's repo as a descendant.

My understanding of the general flow layout for the kernel is:

developer branch -> patch -> mailing list -> sub maintainers -> large maintainers -> Linus -> Stable branch X.X -> distribution (i.e. Debian/etc).

With some things cherry picked here and there that don't flow through this, but that most code does.