installing mods is as easy as putting .pak files into the right folder. thats it.
if you want to create mods yourself, you need to extract the files from the pak, modify them (and bake, if dealing with unreal assets), pack them in a new pak and install that
i opened the ini file and it says that the path to the file you have to edit is \Documents\Rockstar Games\GTA San Andreas Definitive Edition\Config\WindowsNoEditor\CustomSettings.ini
Documents path usually means that ini file would be inside the savegame? extract that with JKSV and take a look.
thats just speculation though, i dont know if that file exists inside the savegame