From looking at Bazzite documentation, ujust is a collection of scripts that run different updaters/etc. It does cover bios updates by running ujust enable-deck-bios-firmware-updates.

From looking at the Bazzite github, this runs a shell script that checks to make sure you have Steam Deck hardware and don’t have the DeckHD replacement screen or 32GB ram mods installed, and then enables jupiter-biosupdate.service. I’m guessing that service is probably using fwupd for updates, but haven’t been able to confirm it yet. If anyone else has some insight I’d be interested to hear it. I know the bazzite founder is active here but I’m going to restrain myself from pinging him and wasting his time over curiosity.