"In Vanilla, I can add a custom loot table in "\.minecraft\saves\New World\data\loot_tables\mctools\entities\loot_table.json", but spigot has data per world folders and if I put it inside the worlds data folder, it doesn't work like it does in vanilla.

I found out in order to change default loot tables, those are located inside the actual spigot jar and requires you to change it every update. "