You had many questions, but source code doesn't need to be on Github to be open source, especially when it's being maintained and developed purely by people hir

You had many questions, but source code doesn't need to be on Github to be open source, especially when it's being maintained and developed purely by people hired on the core os team inside apple. You could ask why it's not on a private github repo, and the answer is that Apple's code predates Github by probably up to a decade, and depending on workflow and privacy reasons being on github is probably useless and/or veto'ed by their legal department. There are so many ways to work on open source. Github is just one tool.