I wanted to add Potion Core Reborn effects to Reliquary items, but I couldn't find anything on it in data folder

There's a potion map in the reliquary config where you can assign effects and their rough duration / amplifier to ingredients. The way this works is that you have to have two ingredients that share the effect and then the effect gets added to resulting potion essence and duration / amplifier is calculated based on what's in potion map. So you should be able to do what you want in there. I will probably convert it to datapack in the near future but it's not there yet.