i don't think steamos is based on arch? pretty sure the whole edifice of steam on linux is based on debian based containers, if you have a steam installation on linux you will find the word "debian" in dozens of folder names in most of the installations

what arch really has over debian based distros though is how easy it is to manage kernels better, and all the nixos loonies rave about their configuration system but Arch is the king of custom installation scripting with AUR, and nobody has really improved on it except making it more complicated and opaque