Specifically, SteamOS for the Steam Frame. I have so many questions. The Frame runs “SteamOS”. Only SteamOS is based on Arch/x86, and the Frame runs ARM/Qualcomm, so surely this is a sort of “SteamOS for ARM”? I understand FEX (ish), are they just running the entire OS through FEX? Arch doesn’t seem to have an ARM version I can find so did Valve do the work of porting Arch to ARM? And if so, where is the source code?
it’s “unofficial” but arch has had an arm port for years: https://en.wikipedia.org/wiki/Arch_Linux_ARM / https://archlinuxarm.org/
Steam has a bunch of code on the page you linked, including some Steam Frame-specific code. Note however that SteamOS is not an entirely free software (open source) operating system; it contains lots of free/libre software but also contains some proprietary parts. They are only required to ship source code for the copyleft-licensed components in it; nothing requires them to actually distribute a complete working distribution for hardware they don’t want to. And, it appears that in the case of their ARM port, they do not currently distribute an image for anything besides their Frame device.