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Creating a Google Sign-In implementation in Android using Kotlin and Jetpack Compose involves several steps. The process begins with setting up a Firebase project and registering the Android application with the project's package name. The "google-services.json" file is then downloaded and placed in the "app" folder of the project.

Firebase authentication is enabled, and the necessary SHA-1 and SHA-256 thumbprints are obtained for certificate configuration. The Web Client ID is also required to request an ID token from Google through Credential Manager.

The UI for this implementation consists of two screens: Login and Profile. Buttons for logging in with Google and logging out are included in these screens. The design of the interface can be customized using Jetpack Compose, ensuring the inclusion of visual elements that indicate user authentication status.

A sealed class called AuthRes is created to handle the results of authentication operations. This class contains two subclasses: Success, which holds the data returned by a successful operation, and Error, which holds a message describing the error. The sealed class ensures that only these two subclasses can be directly inherited, and the out T parameter allows for covariance, enabling the use of AuthRes<String> where AuthRes<Any> is expected.

A CredentialManager class is then implemented within the "data.remote.auth" package. This class includes a getGoogleIdToken() method that performs four steps: creating a CredentialManager instance, attempting to obtain a credential from a previously authorized account, retrying without limiting the request to previously authorized accounts if no credentials are found, verifying the received credential's type, extracting the Google ID token, and returning it using AuthRes.Success or returning an error using AuthRes.Error.

The method handles various exceptions, communicating different failures such as token processing issues or credential request errors. This method obtains the Google ID token but does not initiate a login session in Firebase, which will be handled by the AuthManager.