As far as I know for the new Vulkans layers and dx12 implementation there is a “translation layer” from the old dx implementation to the most updated one. This is the main reason why old games runs faster on Proton than in w7 for the same hw. Even if they were designed for w7 specifically.

Last time I checked this was done during the booting of the game, but i have to admit this was time ago and it could have been changed.