Some of their stuff is on Github, but must of their stuff is on the linked website. The stuff that is open sourced here is mainly stuff like the Mach kernel, the BSD base system utilities, etc. For the most part, I didn't find the stuff here very useful, with the exception of an issue with 802.11x back in OS X 10.5 days. Was nice being able to reference the exact line in the code in the Radar I filed that was causing the issue.