Routing in ASP.net core with Angular

Every webrequest enters the HTTP request pipeline. This pipeline is constructed in the Startup.Configure method. The webrequest traverses the pipeline from top to bottom, the webresponse traverses the pipeline back to the top.

The most basic, simplest middleware would look like this:

app.Use(async (context, next) => {
    // This middleware can manipulate the HTTP response headers,
    // response body, cookies, ...

    // We can decide if the next middleware should be called or not.
    // Sometimes this may not be necessary
    // (eg. serving a Sitemap, or a static file)
    await next();
});

In your case you have the following pipeline:

// This middleware reads the identity cookie and loads
// the information in the HttpContext.User. Finally calls the next middleware.
app.UseAuthentication();

// This middleware does something about routing decisions.
app.UseRouting();

// This middleware evaluates the AuthorizeAttribute to check if the route is accessible with the current Identity (which has been loaded before).
app.UseAuthorization();

// This middleware decides which controller method should be called. If a controller method matches the request url, the next middleware will not be called.
app.UseEndpoints(endpoints =>
{
    endpoints.MapControllerRoute(
        name: "default",
        pattern: "{controller}/{action=Index}/{id?}"
    );
});

// If none of the above middleware has "handled" the request, or more precisely,
// If each of the above middleware has called the next() delegate,
// The webrequest will end up in this middleware.
// As far as I know, the UseSpa middleware will never call the next() middleware.
app.UseSpa(spa =>
{
    spa.Options.SourcePath = "ClientApp";
    if (env.IsDevelopment())
    {
        spa.Options.StartupTimeout = new TimeSpan(0, 0, 80);
        spa.UseAngularCliServer(npmScript: "start");
    }
});