Through plug-ins. The plug-in allows projects to be built using the VR tech made by each company and you need to read over the documentation for specifically which VR setup you want to build for as certain settings may vary like eye height for the player controller. I’m not sure if there is a way to make a game fully compatible with all VR devices, one would have to pick one device unless you are rich and have access to test out your game on all VR devices. I have only worked on one project for the Occulus and never worked with the VIve yet so I can’t say for sure.