i think to do any kind of modding you have to open the source code in the godot editor and then make changes there then export the resulting pck files which the player then insert into the game files.
this does mean only 1 mod can exist at a time
i have not tried this yet but this is what i have found from my research.
I think this tool from hhyyrylainen is what you were talking about
so you don’t need to use the godot editor just this tool