From the title I thought it was a way to emulate a Steam deck on a computer and was a bit confuse at what the use case would be.
You can emulate a Steam Deck easily on any computer. Just boot up the ISO: https://store.steampowered.com/steamos/download/?ver=custom
AKAIK, SteamOS 3 hasn't been released publicly, and this is SteamOS 2 which is Debian based instead of Arch
You can use https://github.com/HoloISO/holoiso though
Or just launch Steam with the command line argument that gives it the Steam Deck UI