Pink³

I am trying to save some data that is individual to every Player (It should work in Single- and Multiplayer), for now a few variables should be sufficient, but it would be favorable to save data from classes. I already figured out that I somehow have to (probably) use the capability system to save to NBT (I don't want to end up designing my own system where I save data to some json files or a databank :/), but I just can't get my head around it, maybe I am missing some OOP knowledge to understand it? I have tried for about a week now, but still don't even really know how to start, I hope you can either give me a few pointers or show me the way, thank you! <3