As Apple is not developing this software in the public, it would not make sense to put it on GitHub or any other code hosting site. The usual approach is that Apple throws bits over the wall where you can pick them up months after the end-user release. And even these code drops will be thoroughly cleaned before, so that for example kernel sources do not contain any trace of support for ARM or anything else related to the iPhone – all of that is strictly closed source.
Software they take in from BSD, other open source projects, or some common Linux tools might also get patched. However, such patches are never actively submitted back to the projects, but only published on this website.
The sources published for macOS on this website are mostly licensed any variant of the BSD/MIT licenses, or the Apple Public Source License, or the GPLv2. Apple avoids the GPLv3 for legal reasons due to its patent clause. That is why they often ship only the most recent GPLv2 release and ignore all later updates, for example GNU make 3.81 or bash 3.2. There might also be some other licenses in the mix, such as the Artistic License for Perl and related software.
Software they take in from BSD, other open source projects, or some common Linux tools might also get patched. However, such patches are never actively submitted back to the projects, but only published on this website.
The sources published for macOS on this website are mostly licensed any variant of the BSD/MIT licenses, or the Apple Public Source License, or the GPLv2. Apple avoids the GPLv3 for legal reasons due to its patent clause. That is why they often ship only the most recent GPLv2 release and ignore all later updates, for example GNU make 3.81 or bash 3.2. There might also be some other licenses in the mix, such as the Artistic License for Perl and related software.