At least on The Mandolarian this is what happened. Everything behind the actors in the camera's frame would be green while the rest of the volume was used to have a lowres lighting reference for the scene. So essentially it would be a moving green screen. The Unreal output was never directly used in the finished show.
If you just watch the show you can see the Volume screens pretty clearly. The transition from the real set floor to the floor in the screen is usually pretty obvious.
