It seems to be more for if you actually want something that makes use of the screen and gamepad input and then you can launch it from the UI.
https://github.com/ValveSoftware/steamlink-sdk

I thought this too, but unfortunately in terms of modding and general use they are very limited, afaik. When I looked into it, it boiled down to: There’s an sdk to develop stuff for it and you can get root access but good luck trying to replace the os or anything like that. That being said, this is what I remember from ~2 years ago, so if it can be customised more now, please let me know. I kinda bought 2 in hopes of being able to do that :D

I just finished Deaths Door and it was a very enjoyable game to play on the go or in bed (steamdeck). Took me about 17 hours to finish it.