For a bit of history regarding OS-X which may answer some of your questions, Amit Singh wrote about the history of NEXTSTEP[1] and also about XNU[2].

BTW, Singh's writings regarding OS-X are exceptionally well researched, detailed, and thorough. I highly recommend them for anyone interested in OS-X/XNU internals.

HTH

1 - http://www.kernelthread.com/publications/appleoshistory/7.ht...

2 - http://www.osxbook.com/book/bonus/ancient/whatismacosx/arch_...