How to create separated controllers?

[/RaspberryPi]

Hello!
I started a new Raspberry Pi Project (my first one) and I want to create my own handheld console.
I got basically everything sorted (for now) : the screen, the battery, etc...
Now I need to work on the controls and I can either:
-Buy one of these mobile controllers that can extend (somewhat expensive, not satisfying to make but it's easier)
-Create my own controller (supposedly cheaper, more satisfying but harder to make)
Now I want to create controllers just like the Wii U gamepad for example or the Switch with it's joycons : the screen in the middle and the 'controller' split in two. I searched EVERYWHERE online but couldn't find anything, I know i have to do something with the GPIO pins on the board but I have no idea what.
Hope I can get some help,
Cheers

submitted by /u/NoobTehEpic
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