It is immutable and has versioned image releases (unlike Arch, which is mutable and rolling release).This adds stability, makes the system less prone to breakage, and easy to rollback (factory reset).
Ans of course, the biggest visible difference is that this boots directly into Steam big-picture mode, where you operate the UI using a controller (although it does support KB+mouse), which makes it ideal to use connected to a TV, like a regular gaming console.
SteamOS is highly customized and managed by Valve. It doesn't have the Arch repos or pacman. Updates to the OS come from Valve in large releases rather than per package. All apps installed either come through Steam or Flatpak.
So it's Arch in the same way Android could be called Linux.
The way SteamOS works is a bit weird. By default the OS partition is read only, you can do whatever in your user directory. But they let you unlock it to make whatever changes you want.
I’m pretty sure that this OS partition gets completely wiped every update though so your customisations will have to be reapplied.
The hardware is just an x86 pc. You can install windows or any other Linux distro on it.
SteamOS itself is designed as a hands off just works forever experience. You can tinker a surprising amount but it’s a very non standard configuration.
Yup it is Arch Linux, that basically boots into Steam client with Big Picture mode. And if you switch to desktop mode you get a standard Linux with KDE Plasma desktop environment.
Ans of course, the biggest visible difference is that this boots directly into Steam big-picture mode, where you operate the UI using a controller (although it does support KB+mouse), which makes it ideal to use connected to a TV, like a regular gaming console.