Sign-In feature in Unity?

Currently, I am trying to create an android social app/game. One of the basic features we need to implement is to match two users and start a game. I know how to make the sign-in feature with JAVA in Android Studio. My question is in the industry, how do people work this out? Do they combine JAVA and Unity together (plug the unity game project into Android studio) or do it entirely in Unity?

Sorry, I am a complete noobie and this question might be too general and ambiguous.

 

Most likely a web server with Https requests send to it...

 

Usually android plugins places in Assets/Plugins/Android folder and you just invoke methods from them.
https://docs.unity3d.com/Manual/AndroidJARPlugins.html

 

Its possible to do in C# via basic Unity functionality and in native language via plugins as well.
(C# to Java is done via JNI in Unity)

I'd suggest looking into C# side though, as that will provide cross platform support out of the box.
Also, there's already a bunch of those solutions available online.

If you're not going to host server on your own, you can look into something GameSparks alike that fits your project.