It sounds like it will use a compatibility layer, and I’m expecting that said layer will likely be tied to Steam such that you can’t just run a non-Steam copy of a game using it even if it’s perfectly playable through Steam. I hope I’m wrong.
Steam deck uses Proton to provide compatibly to non Linux games. You can use Proton with non Steam games. You can install non Steam games on the Steam Deck.
Steam deck uses Proton to provide compatibly to non Linux games. You can use Proton with non Steam games. You can install non Steam games on the Steam Deck.