I have seen some plugin used blah.db to store data and some used NAMEOFTHEPLAYER.yml to store data The question is database or config.yml is better? like faster or use less memony
I'd say a database is useful for it's purpose; a database. .yml and .json files can be used for storage too, but they're more focused on being able to be edited by hand as well, which a -for example- SQLite database does not allow. Every storage way has their own advantages and disadvantages. MySQL allows databases which can be used by more servers at the same time, which could be very useful for networks. It really depends on what you want to use it for.