The actual kernel of XNU (ie. what’s under the hood in macOS, originally by NeXT) is Mach which is a microkernel, and a chunk of FreeBSD is used to provide userland(ish) stuff like POSIX APIs, a UNIX process model on top of Mach, protocol stacks 'n shit.
Mach/BSD 🤓
The actual kernel of XNU (ie. what’s under the hood in macOS, originally by NeXT) is Mach which is a microkernel, and a chunk of FreeBSD is used to provide userland(ish) stuff like POSIX APIs, a UNIX process model on top of Mach, protocol stacks 'n shit.