How to exclude a specific path in ASP.NET Core 5 so that the request is processed by ASP.NET and not by the static files?

Question

So I have a frontend/SPA in Angular and a backend/API in ASP.NET Core 5. After building the frontend, the build files were copied to the backend's wwwroot folder.

I implemented an authentication flow with OpenIDConnect in my backend. This is triggered by the frontend and leads to a backend endpoint. So you will be redirected to the Identity Provider and back to the callback endpoint that ASP.NET provides.

Now when I open the website, ASP.NET serves me the static Angular files and this redirects me to the Endpoint which starts the Login/Auth process. The process end up redirecting me to https://localhost:5001/callback.

The problem now is that ASP.NET is not serving the request through the pre-configured callback path. It will forward the request to the static files so Angular will process the request.

So my question is: Is it possible to exclude specific paths so that they are processed via ASP.NET instead of the static files?

So far I've tried intercepting the request and only forwarding what doesn't have /api or /callback in the path. As described in these blogs/questions:

• https://www.blinkingcaret.com/2018/01/24/angular-and-asp-net-core/
• https://weblog.west-wind.com/posts/2020/Jul/12/Handling-SPA-Fallback-Paths-in-a-Generic-ASPNET-Core-Server
• My SPA files are giving 404 errors when serving the SPA in a branch of the request pipeline. How do I make my static files available to my SPA?

ConfigureServices in my Startup.cs looks like this:

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllers();
    services.AddSwaggerGen(c =>
    {
        c.SwaggerDoc("v1", new OpenApiInfo { Title = "backend", Version = "v1" });
    });


    JwtSecurityTokenHandler.DefaultInboundClaimTypeMap.Clear();

    services.AddAuthentication(options =>
    {
        options.DefaultAuthenticateScheme = CookieAuthenticationDefaults.AuthenticationScheme;
        options.DefaultSignInScheme = CookieAuthenticationDefaults.AuthenticationScheme;
        options.DefaultChallengeScheme = CookieAuthenticationDefaults.AuthenticationScheme;
    })
    .AddCookie(options =>
    {
        options.Cookie.SecurePolicy = CookieSecurePolicy.Always;
        options.Cookie.SameSite = SameSiteMode.Strict;
        options.Cookie.HttpOnly = true;
        options.ExpireTimeSpan = TimeSpan.FromHours(2);
        options.SlidingExpiration = true;
        options.Cookie.MaxAge = TimeSpan.FromHours(2);
    })
    .AddOpenIdConnect(OpenIdConnectDefaults.AuthenticationScheme, options => ConfigureOpenIdConnect(options));

    services.AddControllersWithViews();


    services.AddSpaStaticFiles(configuration =>
    {
        configuration.RootPath = "wwwroot";
    });


    services.AddCors(options =>
    {
        options.AddDefaultPolicy(builder =>
        {
            builder.AllowAnyOrigin();
            builder.AllowAnyHeader();
            builder.AllowAnyMethod();
        });
    });
}

My Configure looks like this:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
        app.UseSwagger();
        app.UseSwaggerUI(c => c.SwaggerEndpoint("v1/swagger.json", "backend v1"));
    }
    else
    {
        app.UseExceptionHandler("/Error");
        app.UseHsts();
    }

    app.UseCors();
    app.UseHttpsRedirection();

    app.UseRouting();

    app.UseAuthentication();

    app.UseAuthorization();

    app.UseEndpoints(endpoints =>
    {
        endpoints.MapControllers();
        endpoints.MapControllerRoute(
            name: "default",
            pattern: "{controller}/{action}/{id?}");
    });


    app.UseDefaultFiles();
    app.UseStaticFiles();
    app.UseSpa(spa =>
    {
        spa.Options.SourcePath = "../../frontend/";

        if (env.IsDevelopment())
        {
            spa.UseAngularCliServer(npmScript: "dev:start");
        }
    });
}

private void ConfigureOpenIdConnect(OpenIdConnectOptions options)
{
    options.Authority = Configuration.GetValue<string>("Authority");
    options.ClientId = Configuration.GetValue<string>("ClientId");
    options.ClientSecret = Configuration.GetValue<string>("ClientSecret");

    // Set response type to code
    options.ResponseMode = OpenIdConnectResponseMode.FormPost;

    // Configure the scope
    options.Scope.Clear();
    options.Scope.Add("openid");
    options.Scope.Add("profile");
    options.Scope.Add("offline_access");

    options.CallbackPath = new PathString("/callback");

    // This saves the tokens in the session cookie
    options.SaveTokens = true;
    options.UseTokenLifetime = false;
}

Update 1:

folder structure of the ASP.NET project:

---

folder structure of wwwroot:

these are the Angular build files. In the assets folder is the font and some images