Hi again, xentidoe! As you know, FNF is an open-source game so anyone can dig into its source code and use it to port a mod of his choice. It takes programming skills though. At first, I export/import a mod's assets (images, sounds, etc.). Then, I go through the source code files (https://github.com/ninjamuffin99/Funkin/tree/master/source) and edit them so that the vanilla game turns into a mod with all the imported assets. There is no strict step-by-step tutorial though. Each mod is unique and takes broad coding skills to get ported.