I don't follow Apple open source, so I have a lot of questions: What parts of MacOS are open source, and which are not? How is it licensed? Why is this on their website rather than GitHub?

> rather than GitHub?

Just to note that Swift is on GitHub[0] and has a very inclusive community.

The impression I get is that Apple isn't really interested in soliciting outside contributions for their OS/kernel code, which is why they're just code dumps.

[0] https://github.com/apple/swift