Ideally you would never have to because you just have the two people come up with their part of the password and then initialise the LUKS partition together. Sorta like a key ceremony

You can also just split the password for a single LUKS into two parts and give one each to the two people :D

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

Honestly my recommendation would be to start off with some microcontrollers/Raspberry Pi and some breadboards and go from there. That way you can uhhh trial and error your way to something that works and worry about the “engineering” part later, when you have gotten your feet wet :D There are some simple Arduino starter kits for example that give you the basic things + a bunch of sensors and motors to play with. I got started myself with the Elegoo Arduino Starter Kit (~50€). Along the way it’s probably going to be helpful to look up some course on electronics basics, specifically digital circuits and then analog circuits if you’re feeling adventurous.