It's also worth noting that Apple could have used a different engine, or started from scratch. Similarly, they could have chosen not to open source the darwin

It's also worth noting that Apple could have used a different engine, or started from scratch. Similarly, they could have chosen not to open source the darwin kernel, and they could have based it on something other than BSD. The point was that Apple seems to know when to choose open source technology and leverage it, and when to start from scratch and build their own proprietary system.