XNU has some interesting features.

It’s Mach based, so it has Mach ports, which are an IPC system that is fairly heavily used by macOS.

It has DriverKit, which is a C++ framework for drivers that’s got morecsuppirt than the usual Unix driver model.

Mach-O does more than ELF, and enables the two-layer namespacing that makes macOS able to manage dynamic linking dependencies better than ELF.

Nothing really earth-shattering but interesting enough. And far easier to build a macOS environment on than FreeBSD.