apple uses a BSD kernel

BSD is a microkernel, it is smaller and puts more things in userland

windows and linux kernels are basically the same in their architecture, giant, monolithic blobs with enormous APIs, as compared to the small and well organised BSD kernel

note that BSD has been a free kernel for a long long time, and yes it was developed by students and postgrads in berkeley university

you know the reason why it isn't more widely used?

hardware.