@Sarcasmo220 Straight from Valve: "SteamOS versions 1.0 [and] 2.0 [...] 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." (https://developer.valvesoftware.com/wiki/SteamOS)

TL;DR - if you're using one of the original Steam Machines, you're running Debian. If you're using a Steam Deck, you're running Arch. (No official upgrade path from v1/v2 to v3 as far as I'm aware.)