How are stats, skills, NPCs and maps stored in RPGs? And how does leveling up work?

For context, I am using the bevy engine.

I know that you aren't supposed to store stuff that might contain a lot of information, such as dialogue, NPCs and skills.

I already found a library that helps me store dialogue (bevy_yarnspinner), but I've yet to find such a library for other ressources.

So I was wondering, usually in RPGs, where and how are stats, skills, NPCs (including enemies) and maps are stored? And how does leveling up work?