Micro kernels and and OS personalities were once a think in Mach. Today, the micro kernel is simply called Linux (ironic since Linux was designed explicitly to not be a micro kernel).
What? Microkernels have a specific definition too, and Linux most certainly is not one.
Yes, that is the irony: Linus wanted Linux to be the anti-micro kernel OS, and it winds up 20+ years later being what people prefer to layer higher level OS's on.
Well, he never intended for it not to be a kernel, which is what higher level operating systems are built on.