This is a good question. Is there documentation that really explains the Quartz architecture somewhere?

The Apple documentation on Quartz 2d is pretty good (although depending on how familiar you are with the rest of the system, you might need to follow some of the links to read about the other parts):

https://developer.apple.com/library/mac/#documentation/Graph...

That's more like Cairo though. I don't think Apple have any public documentation on how the window server part of their platforms works.