Steam isn't a platform, it's an alternate app store. The Platform is PC/Mac/Linux. And they each have their own native app store.

PC has the Microsoft Store, Mac has the App Store, and Linux has it's own distro repositories depending on flavor.

SteamOS is just a tweaked Debian distribution, and you have full control of the os, so you can install any competing software you want.

Yes, yes it does.

There is a menu item labeled 'Add A Non-Steam Game To My Library', after adding you can also use steam's compatibility tools (Proton, or DOSBox or whatever custom compatibility tools) with the games as well.

Windows, Linux, and Mac do. I'm proposing this scoped just to device+OS combo that you purchased.