This tutorial from 1.12.2 is as easy as anyone could possibly explain it, it hasn't changed much til now except some slight changes here and there for you to figure out, it's pretty straightforward (https://www.planetminecraft.com/blog/forge-tutorial-capability-system/). Obviously instead of mana you'd just replace it with whatever else you're doing. All a capability is in essence is the ability to serialize/deserialize standard data types to an entity. When the player dies the game creates a new instance of a PlayerEntity object so your data won't automatically "transfer" over to that new object unless as you say, you copy the data over in an event hook.
After you have this set up, if you want to access the data you store in player capabilities you create a class with a static get function that retrieves it.