Check out example codes for "native-googlesignin configuration is null!?". It will help you in understanding the concepts better.

Code Example 1

/*
         * Uses google signin
         */
        void OnSignInFirebaseGoogleSignin()
        {
            if (GoogleSignIn.Configuration == null) {
                Debug.Log("AUTH Setting Google Default Configuration");
                GoogleSignIn.Configuration = new GoogleSignInConfiguration
                {
                    WebClientId = lifecycleManager.stagingModeToggle.isOn ? stagingWebClientId : productionWebClientId,
                    RequestIdToken = true
                };
            }

            Task<GoogleSignInUser> signIn;

             Debug.Log("AUTH Google Signing in, pop up should appear ");

                // this pops up the google signin.
                signIn = GoogleSignIn.DefaultInstance.SignIn();

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