Created in Unreal 4. Coded in Blueprint
Most of this work is just with cameras and materials. Each side of the cube is a scene that exists. There is a camera tied to that scene. As the player moves around, the cameras in those scenes move around. The capture of those cameras is dropped to a material that is placed on the cube. This allows it to appear as if the character is peering into an open world. For optimization, only the sides of the cube that can be seen are actively capturing data. The sides of the cube out of view are disabled for performance.