Why speculate when you can read directly from the product page yourself?
>You can also install and use PC software, of course. Browse the web, watch streaming video, do your normal productivity stuff, install some other game stores, whatever.
and
>The default Steam Deck experience requires a Steam account (it's free!). Games are purchased and downloaded using the Steam Store. That said, Steam Deck is a PC so you can install third party software and operating systems.
This is pure speculation on my side and I really hope I'm dead wrong but that's the first thing that comes to mind when I think about it (also, their own DRM right?).
Edit : the reply bellow seems to prove me wrong =)