Before diving into Firebase Authentication, make sure you've got these covered:

• Created a Firebase project in the Firebase Console.
• Added Firebase to your app, whether it's Android, iOS, or Web.
• Included Firebase Auth and Firebase Core libraries in your project dependencies.

Head over to your Firebase console and find the "Authentication" section. Click on "Sign-In Method". You'll see a list of sign-in providers.

For example, to enable Google Sign-in, find "Google" in the list, click on it, and toggle it on. Fill in the details for the Web SDK configuration.

For other providers like Facebook or Twitter, follow similar steps and provide the necessary API keys and secrets.

To use Firebase Authentication in your app, you need to add the Firebase Auth SDK. Here's how you can import it:

For Android:

Add these lines to your build.gradle:

implementation 'com.google.firebase:firebase-auth'
implementation 'com.google.android.gms:play-services-auth'

For iOS:

Add this to your podfile:

pod 'Firebase/Auth'

For Web:

Include the following scripts in your HTML:

<script src="https://www.gstatic.com/firebasejs/6.1.1/firebase-app.js"></script>
<script src="https://www.gstatic.com/firebasejs/6.1.1/firebase-auth.js"></script>

After importing the Firebase Auth SDK, you can use FirebaseAuth.getInstance() in Android, Auth.auth() in iOS, and firebase.auth() in web apps to get an instance of FirebaseAuth.

You can use the FirebaseUI Auth library to handle the sign-in flow with minimal code.

For Android:

First, add the FirebaseUI Auth dependency to your build.gradle.

implementation 'com.firebaseui:firebase-ui-auth:6.2.0'

Then, start the sign-in flow like this:

List<AuthUI.IdpConfig> providers = Arrays.asList(
  new AuthUI.IdpConfig.GoogleBuilder().build(),
  new AuthUI.IdpConfig.FacebookBuilder().build(),
  new AuthUI.IdpConfig.TwitterBuilder().build());

startActivityForResult(
  AuthUI.getInstance()
    .createSignInIntentBuilder()
    .setAvailableProviders(providers)
    .build(), 
  RC_SIGN_IN);

Here, RC_SIGN_IN is a constant you've defined. It can be any random integer.

For iOS & Web:

For iOS and Web, refer to the FirebaseUI GitHub repositories for iOS and Web for detailed instructions.

For Android:

Once the sign-in flow is complete, FirebaseUI returns to the onActivityResult method. Handle sign-in success or failure like this:

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    if(requestCode == RC_SIGN_IN) {
        IdpResponse response = IdpResponse.fromResultIntent(data);
        if(resultCode == RESULT_OK) {
            FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();
            // Sign-in successful
        } else {
            // Sign-in failed
        }
    }
}

If the sign-in is successful, you can get user information from the FirebaseUser object.

Signing out a user is simple. Just call FirebaseAuth.getInstance().signOut(). This will clear the currently signed-in user. If single sign-on is enabled, the user will remain signed in on their Google Account outside of your app.

Make sure to guide users correctly in your app, so they understand the level at which they have signed out and the implications it has on their account connections.