SteamOS (the operating system for the Steam Deck) is based on Arch Linux (the blue A), so that’s what’s going on in the bottom panel.
The red swirl is for another Linux operating system, called Debian. I don’t know what OP is referring to by Steam ‘leaving’ Debian in the top panel.
It’s one of the Gnome default wallpapers
https://zebreus.github.io/all-gnome-backgrounds/