The new `git fsmonitor--daemon` is mostly shared code but with custom "backends" for each platform hidden a small API. Yes, the Mac backend is built using FSEvents. The Windows backend uses ReadDirectoryChangesW. In both cases, the backend has a thread dedicated to receiving these events/notifications from the kernel, normalizing them, and queuing them for future client requests.
For sanity reasons, I had to draw the line at two platforms in the initial implementation.
There have been discussions about doing a Linux version using either inotify or fanotify, but that work has not started. (And $dayjob might keep me busy for a while on other things, so I'm open to others picking up that work.) It shouldn't be too hard given the API boundary and threading that I set up.
Likewise, for BSD.