A few things to note.
You can install Windows on it if you want to and it will work like a normal PC.
Valve has been working on Proton since I believe 2018 which is a way to get Windows games to work on Linux. If you use the Steam client on Linux, you shouldn't need to tweak anything to get Windows games to run Linux for the majority of the games. Valve also said they are working with Anti Cheat developers to enable Linux compatibility for Windows games that have Anti Cheat engines. According to Valve, by the time Steam Deck comes out, every game on Steam will run on Linux.
The Steam Deck ships with Steam OS 3.0 which runs a custom version of Linux. As a casual user, if you ever used big picture mode on Steam, it will look and function very similar to that. It should be as simple as logging in, hitting the install button to download/install the game and pressing play. Valve is making it very user friendly but you do have access to the full desktop version of Linux if you are a power user.
I had never heard of proton, I'll check it out. When I got my latest laptop, I was considering going with Linux, one of the aspects I checked was gaming: I checked for playing windows games on Linux, through Wine and there were lots of tweaks to do and games weren't guaranteed to work.
I have my doubts about the seemlessness of the integration but at least it sounds better than I had expected given what you're telling me. We'll see
Some big games yet don´t work on Proton.