It's based on a microkernel (mach) but is hybrid, and the source is available https://github.com/apple-oss-distributions/xnu/tree/xnu-8020...
https://support.apple.com/guide/security/kernel-extensions-s...
Mach is a pre-Liedtke microkernel. It's slow at IPC. Which is bad when IPC is what it has to do the most in a proper multi-server system.
This the the best thing in commercial software, decisions happen, and there are no forks botching the final outcome by disgruntled devs.
I'm not a software engineer, but out of curiosity.
I wonder if the computer industry in general tend to overlook at performance, that's how I feel about the hardware community, where a tiny differences in benchmarks always taking the crown.