SteamOS is a linux distro, similar to any other. It’s a amalgamation of different pieces of software, including a desktop environment (plasma). But it does not boot into the desktop mode by default, instead it boots into their own graphical environment (gamemode) by default, running their steam client.
That’s because their main focus is gaming machines, and that’s why they want gamers to be greeted with a consolized, 10-feet UI.
I think your confused because you think of steamOS being the UI that welcomes you when you boot into it, instead steamOS is the entire package, including the desktop environment (which is KDE Plasma), gamemode, etc.
ChairmanMeow@programming.dev 18 hours ago
KDE Plasma is just the desktop environment. It’s not an OS. SteamOS is a full OS, built off of Arch Linux. It has both a Gaming mode, which looks a lot like Steam Big Picture does these days, and a desktop mode that uses Plasma as the graphical shell/interface. It doesn’t matter OS-wise which one you “boot” into, as both are SteamOS.