SteamOS is an immutable Linux distro that is designed designed to be usable without a keyboard / mouse. By default, it boots you into something that's like an optimized version of Steam's Big Picture mode (but with less compositor / desktop environment overhead).

From this interface you can launch games, modify all your settings, pull up performance metrics, change refresh rate, change your CPU / GPU TDP, enable FSR or other scaling methods, etc. all using just a controller with no assumptions about you having a keyboard / mouse handy.

You *can* launch into a regular desktop environment optimized for keyboard and mouse whenever you need to do certain things on it, but the main focus and value add of SteamOS is the controller friendly interface, and lightweight, resource optimized environment.

By default it uses a different UI and dedicated "window manager" (gamescope), it is not just Big Picture. But you can always switch to a traditional desktop.

By far the biggest difference is that SteamOS is an immutable OS. As far as I understand, any additional package (outside of flatpacks) you install will wiped out on an OS update. So everything is fine if what you need is packaged as a flatpack, otherwise you have to hack around.

Mainly packaging and bundling everything together in one drop-in package for every day users to get access to. I've run Linux for 20+ years now, and gotten steam games working a few times over that period of time. Not hard, not impossible, but is probably more than the average Windows user wants to figure out.

SteamOS is really about bringing that final experience to the most number of people, and I am really supportive of their efforts!

That's basically it, just easier. SteamOS is like Ubuntu, comes with bunch of stuff integrated and tested together for a easier end-user experience, but you could build your own Ubuntu, starting with a base Debian install. Same with SteamOS with Arch (or Manjaro which I think is what they actually use?)

Mostly there isn't any difference. I run steam just fine in Mint, it works about as well as my Steam deck, which is to say very well. I guess there is some optimisation and default settings more aligned to the purpose on Steam OS.

Pretty much just the UI. SteamOS is better suited for handhelds and TV screens and wherever you don't want a full desktop experience, just a game launcher.

Not much different, but there is steam exclusive mode, essentially straight to big picture and nothing else.