Any of the VR Virtual Desktop apps are certainly done in UnrealEngine or the like, but I think it's really just a virtual display for your computer and the 'desktop engine' is still windows if I'm interpreting your comment correctly.

Take a look at the Godot subreddit. I've seen at least one or two people who made desktop tools with it.

The way any of the modern compositors work is essentially the same.

Godot editor, a desktop app, is made in Godot itself afaik