https://help.steampowered.com/en/faqs/view/1B71-EDF2-EB6D-2BB3
SteamOS runs a custom grub to cater it's needs for it's A/B partition design. So far the said custom grub does not boot for me on ovmf/intel uefi (maybe it boots on amd? some said they managed to just rufus the image and boot it)
skip to https://gist.github.com/Kethen/698cfa8cf387e131ebd36fbfbfe9272e#gamescope-and-non-amd-gpu if it just boots for you
Upon closer inspection the official bootloader does load some kind of amd firmware before booting the kernel