How to implement cookies, sessions and database connection?

Question

Java

Please share the documentation!
Or explain on your own.
In general, I am creating an Android application in which, according to my idea, there will be authorization / registration and a personal account. For this, as I understand it, I need to work with cookies, sessions and a database connection.
I searched for everything separately and was only able to work awry with the database using Retrofit2. The case with cookies broke off because I did not find almost any normal documentation on okhttp3 and as a result I got confused if I need any other servlets, and in general, what is going on? I'm asking a question here in the hope of getting sources or other information from knowledgeable people, because I can't connect everything on my own. You can offer sources in English!
The usual documentation for example on developer android does not help me, since there are no illustrative examples there, but only a description of how the methods work.