How to make API request from Google authentication in ASP.NET? - asp.net-mvc

Question

I do not understand how I can get the authorization code/access token to make a request.
This is my .AddGoogle():
builder.Services.AddAuthentication(o =>
{
o.DefaultChallengeScheme = "Google";
o.DefaultScheme = CookieAuthenticationDefaults.AuthenticationScheme;
}).AddCookie().AddGoogle("Google",options =>
{
IConfigurationSection auth = builder.Configuration.GetSection("Authentication:Google");
options.ClientId = auth["ClientId"];
options.ClientSecret = auth["ClientSecret"];
options.CallbackPath = "/Home";
options.AuthorizationEndpoint += "?prompt=consent";
options.SaveTokens = true;
options.Scope.Add(FitnessService.Scope.FitnessActivityRead);
});
So I log in / register, and then what?
I've tried this method
UserCredential credential;
var flow = new GoogleAuthorizationCodeFlow(new GoogleAuthorizationCodeFlow.Initializer()
{
ClientSecrets = new ClientSecrets()
{
ClientId = configuration.GetSection("Authentication:Google")["ClientId"],
ClientSecret = configuration.GetSection("Authentication:Google")["ClientSecret"]
},
Scopes = new[] {
FitnessService.Scope.FitnessActivityRead,
FitnessService.Scope.FitnessActivityWrite,
FitnessService.Scope.FitnessSleepRead,
FitnessService.Scope.FitnessSleepWrite
},
});
var token = new TokenResponse
{
AccessToken = "",
RefreshToken = ""
};
credential = new UserCredential(flow, "user", token);
FitnessService fitnessService = new FitnessService(new BaseClientService.Initializer()
{
ApplicationName = "Exrecise App",
HttpClientInitializer = credential
});
var resp = await fitnessService.Users.Sessions.List("me").ExecuteAsync();
But how do I get the tokens to begin with?
I even tried HttpClient (by pasting the token from OAuthplayground)
HttpClient http = new HttpClient();
var resp = await http.SendAsync(new HttpRequestMessage()
{
RequestUri = new UriBuilder("https://www.googleapis.com/fitness/v1/users/me/sessions").Uri,
Headers =
{
{ "Authorization","Bearer " }
},
Method = HttpMethod.Get
});
return Ok(await resp.Content.ReadAsStringAsync());
Same problem, how do I get the access token from the currently signed in user?
I looked everywhere, but every solution is either outdated or is unclear or just doesn't work for some reason.