Help with /item modify

I have never touched anything related to loot tables before and I tend to stray away from commands that use .json files but for a project I'm trying to make, I want a function which changes the nbt data of the player's selected item, in this case I want to change the damage, CustomModelData, and attribute modifyer attack_damage but I don't know how to do it. I found this but I still don't quite understand how to use it.
Can someone please either explain how /item modify works or just help write out the function or .json that allows for me to change the CustomModelData, attack damage, and damage in one command?