What is the principle of authorization in social applications. networks?

Question

Mobile development

Let's take any application of any social network. networks (OK, VK). After successful authorization, we, of course, go. After closing the application and restarting the application, the application no longer requires a login and password - the login occurs automatically ...
So, where are these login and password saved ... i.e. so you don't have to log in again. What is the general principle?
To the cache?

Answer 1

Cookies

Answer 2

to token