There is two ways to use a path.
Using the “file(abc/def.json)” it will always point to your own mod automatically. So you don’t need to inform it or adding the first slash for root. It always loads relative to where the file is, btw. So when using it in the manifest it is always relative to the mod folder.
The other way is using “your_mod/abc/def.json”. So you need to say which is the mod at the start. It is the absolute path, starting at the mods folder.