[iohead]

No, not quite. The macOS/iOS kernel is extremely Frankenstein-y (not meant in a derogatory way), with the majority of codebase being extremely Apple-specific, and bits and pieces originally taken from Mach and BSD. In particular, there is no microkernel, and there never was. Mach itself was never used as a true microkernel in a commercial setting, with the first such implementation--Mach 3--showing significant real-life performance problems. As such, there is no "BSD on top of a Mach microkernel". It is and has always been a fully monolithic kernel with some subsystems originally derived from Mach (Open Group's Mk 7.3), some from BSD (FreeBSD 5), and the rest developed in-house over the years. Even the layerings aren't always clean, with "on top of" often morphing into "alongside" or "intertwined with".

[detourdog]

What is interesting about Mach to me. Is that now 40 years later it's returning too it's roots of hosting multiple OS's on a single hardware architecture. Apple being able to design to M series chips to match Mach's paper over Mach's deficiencies and leverage it's strength I find very exciting.

Originally it was meant to be the foundation computing layer for a campus full of devices.