> Maybe instead of Git you mean GitHub

Without involving github at all, you can use git directly to push branches to a remote repository and then have it be merged by the maintainers there, which leads to a workflow that's pretty similar to the one you have with github.

Instead, Linux is being worked on by exporting the (git) patches and sharing them on a mailing list, effectively not using a significant feature of git and leading to a very different workflow than the one everyone else is using (github users and others alike), so I still find the question legitimate.