>>273807234
Games work on other headsets because 1. they implemented and included both APIs, so when you play a game with an Oculus headset, it'll detect that and launch the game using the Oculus API, or 2. they implemented the SteamVR SDK only, which Valve have designed so that when the runtime receives API calls, it translates them to Oculus API calls, and sends that to the Oculus runtime. The SteamVR runtime acts as a translation layer between the game which was built with SteamVR and the Oculus runtime.

>>273807343
The headset does. The headset has a driver, the driver sends communicates with the runtime using an API. In the case of Oculus, they use their own API, and in the case of SteamVR, they also use their own API, but the difference and why SteamVR works with Oculus is that Steam specifically implemented a mode that translates the API calls their runtime uses to the API calls that Oculus uses. 


And OpenXR is the API that Valve and Oculus partnered together to develop, that will hopefully replace their old APIs so that none of that translation nonsense has to be done and no additional effort has to be done to make a game compatible with different runtimes.