So FSKit is public API and supports userspace filesystems but on macOS most built-in filesystems are kernel modules. On iOS everything except APFS is userspace.

So FSKit is public API and supports userspace filesystems but on macOS most built-in filesystems are kernel modules. On iOS everything except APFS is userspace. Modern Darwin is mostly a monolithic kernel, though more and more things are moving to userspace where possible (eg DriverKit). One interesting side-effect of various spectre mitigations is silicon improving the performance of context switches. That has the side-effect of decreasing the cost of kernel/userspace transitions. It isn't nearly as expensive as people still believe - though it isn't free either.