I was trying to find a solution to my question: https://gamedev.stackexchange.com/questions/187679/how-to-add-a-mob-to-minecraft-1-16. but I didn't get any answers nor comments on the question so I would like to put it in other words:

In Minecraft Java, I am creating many resource packs by myself (no studio or external resource) so, I want to ADD a new file/entity/element/items/etc. to the game to not replace an old item because EVERY item/effect/entity has SOME use in the game. In conclusion: is there any way that I can ADD [not replace] a file in the game and give it custom physical and mental properties? if so, how do you/I

In Minecraft Java, I am creating many resource packs by myself (no studio or external resource) so, I want to ADD a new file/entity/element/items/etc. to the game to not replace an old item because EVERY item/effect/entity has SOME use in the game. In conclusion: how I can ADD [not replace] a file in the game and give it custom physical and mental properties?