SteamOS

SteamOS is an Linux distribution created by Valve to run Steam games without any extra software. SteamOS versions 1.0, released in December 2013, and 2.0 (released in 2015) were based on the Debian distribution of Linux with GNOME desktop, while the SteamOS 3.0 is based on Arch Linux distribution with KDE Plasma 5. It will run on the Steam Deck (version 3.0 or later only) but you can also install it on your home-built computer and turning it into a Steam Machines, or you can just buy a Steam Machines systems.

Screenshot of SteamOS 3.0, with the default KDE Plasma desktop, running on Steam Deck.

More information can be found on this page.