It does yes. Although it launches Steam directly as its own … “shelll”? Is that the right word? KDE is bypassed entirely unless you launch “Desktop Mode”
Anyways, I still wouldn’t recommend Arch to a new user, go with something easier and more mainstream for your first Linux experience. PopOS, Mint, Fedora, Norabora, Ubuntu/Kubuntu
Also, saying Steam Deck uses Arch isn’t wrong, but it’s a bit misleading. It uses an Arch base , curated, configured and tested by Valve, and finally periodically shipped as updates using immutable root images (on a single well defined hardware platform). If you install vanilla Arch yourself you’re responsible for all configuration and testing yourself.
Yes, this happens automatically for me when I launch games. I don’t remember doing anything special to set it up (Kubuntu with nVidia drivers on X11). I do mostly game in true full screen though, not “full screened window”