VR - different apps for different devices?

Hello,

I'm quite new to the world of VR, never experienced it, I wondered what this meant - an app being made available for Oculus or for Vive or GearVR? If it IS indeed, a problem, that an app made for one particular device will not work on another, surely the solution lies in shifting to the *web*, ie. WebVR?? That way, you just make <whatever> once, and it's cross-platform from get go....? Can someone explain the whole thing to me??

Please help.

I didn't find the right solution from the Internet.

References:
forums.cgsociety.org/showthread.php?f=2&t=1489487
Medical Device Marketing Video

Thanks