More or less Wine + some experimental patches not yet I twgrated in mainstream wine + a buch of DirectX translation libraries + close steam integration.

There's also Proton-GE [1], which is even more experimental and adds some bleeding edge fixes and features.

I've heard it's pretty good for fixing video playback/rendering (e.g. cutscene) issues if both the stable and the experimental branch of Proton can't make it work.

[1] https://github.com/GloriousEggroll/proton-ge-custom

Though currently Proton has not yet shipped a release which uses Wine 11.

That makes sense. I thought they were entirely separate tbh but it makes sense that they'd share a lot of DNA.

I absolutely love my Ally running SteamOS. Incredible work by... everyone involved, really.