Android client site. How to check if a user is actually logged in to an external service using oAuth?

Question

Android

How oAuth on the site's Android client interacts with the site. Throw a link at me) I
don’t understand anything yet. It turns out the user from the client moves through the data. It comes across an authorization request, logs in using a third-party service and says -
"Site, I'm like, such and such on g +" ... on the basis of what the site should believe it?

Answer 1

accessible and understandable