How to implement user communication on Android with the server?

Question

Java

I am writing a simple application that will communicate with a database on a server. I have some questions.
1) Let's say the user is logged in. Where should I write about it, to a file? If in a file, then what to save the login / password? The user has data, should they be stored in a file, or should they be downloaded from the database every time?
2) How to access the database? I heard something about the token, but we don't have any information

Answer 1

I would add two more words oauth and rest to the word token.
Nobody stores passwords on the device.
after user authorization, the application goes
to the server API created on the rest architecture with a token
and receives the necessary data