SteamOS 2.x was based on debian, but that hasn’t been updated in years. The Steam Deck launched with SteamOS 3, which is actually built on top of arch and is much more akin to Manjaro. As for your question, it’s mostly the “game mode”, which uses IIRC Wayland and wraps games inside gamescope which provides a bit more control in the form of controlling frame rate, resolution, etc externally, but regardless, that can and has been achieved in custom distros. I think the main appeal of SteamOS honestly is the package of an immutable OS optimized for running games on steam. It prevents non-linux users from breaking things and tries to make it feel more like a “console” with a “desktop mode” (KDE Plasma) and “app store” (ala flatpaks). I’ve toyed with the idea of running it or similar on my gaming PC but always run into the difficulty of Nvidia drivers on Linux.
Yeah, I’ve gotten Nvidia drivers working, but always with issues in performance or things like Wayland not working and I eventually decide “that was fun to play around with, back to Windows”. Specifically the whole immutable OS thing for me is nice too. Linux is my OS of choice for development work, but it’s so easy to screw things up when trying to make games work nicely and my ideal “it just works” gaming machine can ignore the OS entirely (looking at you, Windows random forced reboots or virus scans), so having big single updates rather than individual packages and no risk of trying to get WINE etc up and running breaking something with the OS is nice. Not to mention, it’s much harder for a small child to brick it than a normal Linux distro or even Windows for that matter. Fedora Kinonite is on my radar for a future distro because of this, I just haven’t decided if I prefer the Fedora ecosystem over Ubuntu or Manjaro.