When you subscribe to a mod on Steam Workshop, Steam downloads a folder (e.g., mod_1234567890 ) into that directory. Your goal is to replicate that process manually. This is the trickiest part. The largest repository is still Steam Workshop, but you cannot download directly from Steam without owning the game on Steam (and launching it).
Without Steam, you have to track these dependencies down manually. the binding of isaac repentance mods no steam new
This folder exists regardless of where you bought the game. Steam creates it. Epic creates it. GOG creates it. Even a portable copy creates it. When you subscribe to a mod on Steam