You can just launch them like normal and have Steam open in the background. Or launch them as non-Steam games from Steam if necessary. They generally detect your headset and controllers.
I dont know that you can play pirated VR games for a different platform that easily though, as in launching a Quest exclusive game with an Index or vice versa. I never tried.
There is also OSSU which has been around a lot longer. https://github.com/ossu/computer-science