Absolutely. But they want everyone to know they are using Windows, which if there was a custom shell then they wouldn’t.
They have >95% of the market, they don’t care about making a good interface for TVs as they already lost it to Android, and they don’t care about making a good gaming interface as they already have XBox.
Windows users are an inconvenience, that they want to milk.
No, Microsoft don’t allow it. As part of the distribution licence you are not allowed to customise the OOB interface.
People don’t know it’s Linux, but it’s absolutely because of Linux that it works.
They could call it Windows Console Edition, or WinCE.
Nope.
The big thing is that the Deck uses Linux which allows Steam to provide an amazing interface.
All the “competition” still tries to use Windows, and the experience is appalling.
They’ve also got the Xbox interface for controllers, the reason they don’t make Windows better for this scenario is that they already have XBox and making a SwitchBox interferes with their plan to make everything shit.