Calling it an OS is a bit generous—although XNU is open-source, actually getting it running on hardware from just the publicly released parts is not particularly feasible. PureDarwin exists, but it's mostly just a curiosity to run in QEMU at this point.

It is, but it's also a complicated story that doesn't seem to go well. Basically past honeymoon period, the open source part of the project just was increasingly left on the side of the road.

https://en.wikipedia.org/wiki/Darwin_(operating_system)

And yet it is not possible to run a single MacOS targeted GUI application on it without many proprietary components on top.

Since macOS is / was a port of xBSD, how much difference is there between Darwin and xBSD kernel?

The userland was largely BSD, but the Mac kernel was a Mach kernel w a BSD shim for syscalls, iiuc.

A significant part of the FreeBSD kernel was used in XNU.