Technically they are using (abusing?) a container containing Windows 11.

https://github.com/basecamp/omarchy/blob/master/bin/omarchy-...

The container they spin up: https://github.com/dockur/windows

And vanilla Arch is still in play, the only thing they do is a shit ton of configuration so you don't have to do it yourself. Basically dotfiles but on steroids.

It's also great for some inspiration if you want to do some ricing yourself.