I was curious and installed it and a small game. It uses Proton GE. But for the life of me I cannot find where it stores the actual game. Supposedly it should write to ~/Games but it's not in there. Searching for the executable doesn't yield anything either. But it still works.

Edit: No idea what witchcraft this is. When the game is running ps shows that Proton is running an executable located in ~/Games but nothing is there.

Edit 2: The sloppers didn't give the flatpak permission to write to the Games folder. Interesting that the game still didn't show up anywhere. Guess flatpak stored the files in memory.