Android app login & register

Hi,
I create my app for android.
The application has the latest posts. This is to ensure that only those who are logged in and have subscribed (level 1) are allowed to view. how to achieve it The plugin I have installed and configured (after the website everything works).

There is no additional login procedure, s2M uses the usual WP login. So if you can get WP login working, you are safe.

How can I easily add a wordpress login to an application?

You are asking a WordPress question, not one related to s2Member. So try Google.

You will probably need to use the REST API but, as I say, that has nothing whatsoever to do with s2Member.