Xbox doesn't provide a game engine, just lower level APIs. Unity is a popular way to get games on Xbox for folks who can't build their own engine. Further, things like HoloLens run Unity as a first class dev environment.
I used to work on both Xbox and HoloLens at Microsoft, but this is all public info.
I used to work on both Xbox and HoloLens at Microsoft, but this is all public info.