I'm not familiar with SteamOS and Valve hardware in general. Could I play something like Overwatch on this, and connect keyboard and mouse? Could I play other PC games like World of Warcraft?
Linux more or less runs most Windows games. The ones that don't run are ones where the developer is going out of their way to make them not run - mostly with kernel-mode anti-cheats that just find themselves staring at the wrong kernel.
Steam makes that pretty seamless and Steam games "just work". For non-Steam games you need to do some tinkering, it's stuff that most people browsing this forum can do.
I've run Bazzite on my desktop for the last year and every update has just been hitting the "Apply" button in the settings page with my xbox controller. While on mutable distros it's always involved going in to the terminal and running a series of commands or opening the repo list and manually replacing the release name for Debian. I know there is a GUI software store to do it but it literally never works because some error will show up that isn't handled and you just get a generic error message.
It follows a different philosophy. I've been using atomic systems for the past year or so as my main driver.
If you want to install something that needs superuser access, you do it inside a container. This protects your OS from breaking.
The number of times I've accidentally installed something which broke my window manager or compositor is now zero