True. But git repositories can also be stored many other places. I think I have at least half a dozen at home for my personal software, and they are only stored on my home server, and nowhere else (in particular not on GitHub). Matter-of-fact, because of git's distributed nature, there is typically not one "server" repository and many "clients", but repositories are equivalent. Anyone who downloads the complete source will have a repository on their own computer. And in theory could re-distribute their own repository to other people, and start a whole separate tree. However, there is typically or a few repositories from which official builds are made.* Github stores git repositories.
True. But it is only one of many copies. I could make a copy of the source (honestly, I've rarely downloaded the FreeBSD source in my life), and modify it to my heart's content, and redistribute it, if I felt like it.* FreeBSD has a copy of their repository on Github.
That's the copy from which the official builds are made, and which the official developers work on.* The master copy of the FreeBSD repository is not on Github.