It's Wine + DXVK (including D9VK since they merged together) + ESync with a different Wine environment for each game. Valve also helped Codeweavers developers to implement controllers support thru Wine.
> ESync with a different Wine environment for each game
This is where Proton was a win for me. It's not super hard to make many of the popular games work, but managing the different wineprefix's or making sure some random DLL is or isn't installed was a pain. Proton keeps 'em clean, separate, and up to date.
