It’s an immutable filesystem fork of Arch, not vanilla Arch. It’s as stable as any operating system is, doesn’t update in the same way Arch does, and doesn’t even allow the user sudo privilege. Also, you don’t even need to interface with Linux. The front end is extremely intuitive, but you also have the option of a pretty great KDE Plasma desktop mode if you switch over. Fortunately, it has no Candy Crush, Teams, or Cortana in the Start Menu.
Not out of the box. You don’t have sudo privileges on the SteamOS 3. Valve made an immutable fork of Arch for stability and to dissuade unskilled folks from punching stuff into the terminal that might cause customer service nightmares.