委派执行顺序

Question

我正在学习代理异步和等待。我得到了委托的想法，它就像一个函数指针，可以很好地理解简单的委托。我也在做一些实用的lambda。如果指出我从一个站点获得的代理中的执行流程，我将非常感激。

using System.IO;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Configuration;

namespace Builder.Filtering.Web
{
    public class Startup
    {
        public void Configure(IApplicationBuilder app)
        {
            app.Use(next => context => FilterAsync(context, next));

            app.Run(HelloWorldAsync);
        }

        public async Task FilterAsync(HttpContext context, RequestDelegate next)
        {
            var body = context.Response.Body;
            var buffer = new MemoryStream();
            context.Response.Body = buffer;

            try
            {
                context.Response.Headers["CustomHeader"] = "My Header";

                await context.Response.WriteAsync("Before\r\n");
                await next(context);
                await context.Response.WriteAsync("After\r\n");

                buffer.Position = 0;
                await buffer.CopyToAsync(body);
            }
            finally
            {
                context.Response.Body = body;
            }
        }

        public async Task HelloWorldAsync(HttpContext context)
        {
            context.Response.ContentType = "text/plain";
            await context.Response.WriteAsync("Hello world\r\n");
        }

        public static void Main(string[] args)
        {
            var config = new ConfigurationBuilder().AddCommandLine(args).Build();

            var host = new WebHostBuilder()
                // We set the server by name before default args so that command line arguments can override it.
                // This is used to allow deployers to choose the server for testing.
                .UseKestrel()
                .UseConfiguration(config)
                .UseIISIntegration()
                .UseStartup<Startup>()
                .Build();

            host.Run();
        }
    }
}

问题

我的理解是，首先调用FilterAsync，然后调用HelloWorldAsync。但是如何知道何时致电HelloWorldAsync？另外，await next(context);的作用是什么？

我不是在寻找对.net核心的深刻理解。这只是了解委托流程的一个示例。