Linux

In regard to question one: it depends. Pretty much everything without a shitty, Kernel-Level Anti-Cheat (my autocorrect corrected to antichrist — for good reason!) will run either by default on steam or with something known as Proton. But you still may run into occasional difficulties.

For example, if you play Counter Strike 2: up until January this year, playing on Linux meant ≈20% less performance (CS2 is unoptimized for Linux and Vulkan unfortunately); this number has changed since the last few updates and since the new Nvidia driver, so I need to re-run the benchmarks. Your going to occasionally experience things like that, where performance isn't on par. In the case of CS2, the devs love Linux, so they will optimize for it in the future. It's just going to take a while.

Another example: I had to use Proton on a game that supposedly was native to Linux. Native implementations may sometimes suck; the good news though, is that you can easily use Proton, both inside and outside of steam. Seriously, I freaking love Valve for Proton, it's a fantastic tool.

This is all to say, that while gaming is absolutely possible nowadays, you will occasionally need add some flag, or familiarize yourself with proton, etc.

The exception, of course, being Kernel antichrists. Goddamn them. I can't play LoL anymore because of it. Well, I hate Riot so much now anyway, I'm not sure I'd want to anymore.