I saw that this program was made in Godot and I've been trying to figure out how they were able to implement global hotkeys for the hotkey feature in the program. Did they make their own addon? Is there an existing addon they used? Is it C# libraries? I just wanna know how.

the background hotkeys are implemented using a gdextension addon (c++) specifically made for pngtuber plus by Horobol.

pngtuber plus's source code is available here. inside the bin folder you can find the compiled gdextension if you wish to use it for your own project. 

the source for the gdextension itself is here, although im not sure if this repo is entirely up to date with the version inside pngtuber plus