How is authorization without a username and password arranged?

Question

Swift

The user enters the application (for example, a game), does NOT enter any data, goes through several levels. How to make it so that when this user logs in again, the game does not start over from the first level?
In many applications this is implemented (without authorization and login), I can not find an answer how

Answer 1

The idea is that the server passes a unique value to the client.
The client saves the value, (cookies, storage, db ..) On the next session, the client passes this value to the server. The server on its basis identifies the client. That's how the sessions are.