It’s a lot easier to lean to use a multi meter to check currents and resistances on an existing board - especially if you have the documentations shown in his videos. This is also why they fight to get them form the manufacturer.
Next you need to learn to solder. I can recommend to build your own keyboard from a kit.
As an Emacs user I prefer split keyboards like the ergo dox, Lily58, Corne, with thumb clusters to, so I can put CTRL, ALT, from my pinkies to my thumbs.
You will also learn to program the controller with firmware and can dabble with that as you need to burn the layouts on your new board.
Lastly you can start to design and build your own bards - which is still low complexity EE and thus a good entry point.