Step 1: Head over to the Firebase Console and either create a new project or pick one you already have.

Step 2: Add your Android or iOS app to the Firebase project. Follow the instructions to download the google-services.json (for Android) or GoogleService-Info.plist (for iOS) file and pop it into your project.

Step 3: In the Firebase Console, go to the Authentication section and turn on the methods you want, like Email/Password or Google Sign-In.

Step 1: Open up the build.gradle file for your app-level module and add the Firebase dependencies:

dependencies {
    // Firebase Authentication
    implementation 'com.google.firebase:firebase-auth:latest-version'
}

Step 2: Sync your project to make sure all the dependencies are good to go.

Step 1: In your app's manifest file, add the permission needed for biometric authentication:

<uses-permission android:name="android.permission.USE_BIOMETRIC" />

Step 2: Add the biometric dependencies to your app's build.gradle file:

dependencies {
    // Biometric Authentication Dependencies
    implementation 'androidx.biometric:biometric:latest-version'
}

Step 3: Sync your project again to make sure everything is integrated properly.

Step 1: In your activity or fragment, import the necessary biometric classes:

import androidx.biometric.BiometricPrompt
import androidx.core.content.ContextCompat

Step 2: Create an instance of BiometricPrompt:

val executor = ContextCompat.getMainExecutor(this)
val biometricPrompt = BiometricPrompt(
    this,
    executor,
    object : BiometricPrompt.AuthenticationCallback() {
        override fun onAuthenticationError(errorCode: Int, errString: CharSequence) {
            super.onAuthenticationError(errorCode, errString)
            // Handle error
        }

        override fun onAuthenticationSucceeded(result: BiometricPrompt.AuthenticationResult) {
            super.onAuthenticationSucceeded(result)
            // Handle success
            authenticateWithFirebase()
        }

        override fun onAuthenticationFailed() {
            super.onAuthenticationFailed()
            // Handle failure
        }
    })

Step 3: Set up the Biometric Prompt info:

val promptInfo = BiometricPrompt.PromptInfo.Builder()
    .setTitle("Biometric Authentication")
    .setSubtitle("Log in using your biometric credential")
    .setNegativeButtonText("Cancel")
    .build()

Step 4: Show the biometric prompt:

biometricPrompt.authenticate(promptInfo)

Step 1: Implement the authenticateWithFirebase function, which will be called when biometric authentication is successful:

private fun authenticateWithFirebase() {
    val user = FirebaseAuth.getInstance().currentUser
    user?.let {
        // User already authenticated with Firebase
        // Proceed with your app-specific logic
    } ?: run {
        // Handle the case where the user is not logged in to Firebase
        // For first-time login, you may need to sign in with the chosen method (e.g., email, Google sign-in)
    }
}

Step 2: If it's the first-time login, implement the logic for Firebase authentication, like using Firebase email/password or any other configured provider:

private fun firebaseSignIn(email: String, password: String) {
    FirebaseAuth.getInstance().signInWithEmailAndPassword(email, password)
        .addOnCompleteListener(this) { task ->
            if (task.isSuccessful) {
                // Sign-in succeeded
            } else {
                // Sign-in failed
            }
        }
}

Step 1: Use Firebase to sign in a user once biometric authentication is verified.

Step 2: Store user credentials securely, like with encryption, and use them for Firebase sign-in once biometric verification is complete.

override fun onAuthenticationSucceeded(result: BiometricPrompt.AuthenticationResult) {
    super.onAuthenticationSucceeded(result)
    // Retrieve securely stored credentials
    val email = retrieveEmail()
    val password = retrievePassword()
    // Sign in with Firebase
    firebaseSignIn(email, password)
}

By following these steps, you can combine Firebase Authentication with biometric authentication to give your users a secure and smooth experience.