ChatGpt and Gemini both are loaded with the old information and hence I was not able to get the answer initially, but after having a good discussion with Gemini (after all it is Google LLM ha ha ha and I am trying to use Google OAuth), Gemini took me in the right direction. Here is the way I have implemented it and it is working all right for me now.

I am using .Net 9, MAUI for Android
Nuget package : Duende.IdentityModel.OidcClient (6.0.1)

public sealed class GoogleLoginService
{
    const string clientId = "Your-client-id";
    const string redirectUri = "com.company.app:/oauth2redirect"; // You can set the first part as your android package name
    const string authority = "https://accounts.google.com/o/oauth2/v2/auth";
    const string scope = "openid profile email";
    const string userInfoEndpoint = "https://openidconnect.googleapis.com/v1/userinfo";

    public async Task<string> PerformOAuthLogin()
    {
        try
        {
            var browser = new WebAuthenticatorBrowser();
            var options = new OidcClientOptions
            {
                Authority = authority,
                ClientId = clientId,                    
                Scope = scope,
                RedirectUri = redirectUri,
                Browser = browser,
                ProviderInformation = new ProviderInformation
                {
                    IssuerName = "accounts.google.com",
                    AuthorizeEndpoint = authority,
                    TokenEndpoint = "https://www.googleapis.com/oauth2/v4/token",
                    UserInfoEndpoint = userInfoEndpoint,
                    KeySet = new JsonWebKeySet()
                },
            };

            var oidcClient = new OidcClient(options);
            var loginResult = await oidcClient.LoginAsync();

            if (loginResult.IsError)
            {
                throw new Exception($"Failed to authenticate with Google: {loginResult.Error}");
            }

            return loginResult.AccessToken;
        }
        catch (Exception)
        {
            throw;
        }
    }

    

    class WebAuthenticatorBrowser : Duende.IdentityModel.OidcClient.Browser.IBrowser
    {
        public async Task<BrowserResult> InvokeAsync(BrowserOptions options,
                                       CancellationToken cancellationToken = default)
        {
            try
            {
                WebAuthenticatorResult result = 
                    await  WebAuthenticator.AuthenticateAsync(
                                          new Uri(options.StartUrl),
                                          new Uri(options.EndUrl));
                
                var url = new RequestUrl(redirectUri)
                        .Create([.. result.Properties]);

                return new BrowserResult
                {
                    Response = url,
                    ResultType = BrowserResultType.Success,
                };

            }
            catch (TaskCanceledException ex)
            {
                return new BrowserResult()
                {
                    ResultType = BrowserResultType.UserCancel,
                    Error = ex.Message
                };
            }
        }            
    }
}

There could be better ways to do it but for the time being I am going ahead with this approach.