Just a small detail, in Linux, the OS doesn’t handle this kind of thing, for SteamOS this is probably handled by Wayland or KDE, can’t say for sure as I’m not actually very up to date on how much of the desktop management stuff was offloaded to Wayland.
They’re not. I have multiple Linux installations that have no Wayland installed and some don’t even have X installed. The OS which is usually called GNU/Linux is the Linux kernel plus a set of GNU tools that allows you to use the system. No Linux user who knows what they’re doing would call Wayland part of the OS.