How does user registration/login work in apps?

Question

Android

How applications register a user. For example, Vkontakte, we register using a number and password, then where is this data stored?
How does the application remember the user who has already logged in and does not require re-entering the number and password, but immediately directs them to the news screen?
I can not understand the general principles of registration and authorization.
For example, if I want to create my own registration process in the application, without using "login with Google, Facebook, Vkontakte". What is the right way to implement this, taking into account all the security rules, i.e. the way real applications do it?

Answer 1

mobile development is not involved at all here, since registration takes place on the web.
if it’s simple: the application simply addresses everything to the server, in response from which it receives a certain token and uses it for all subsequent requests, based on this token, the server decides whether you have access or not.
in general: welcome to an exciting journey through the world of authentication / authorization.