MediatR generic dispatch

Program.cs

using System;
using System.Threading;
using System.Threading.Tasks;
using MediatR;
using Microsoft.Extensions.DependencyInjection;

namespace ConsoleApp3
{
    public class Program
    {
        private static async Task Main(string[] args)
        {
            var services = new ServiceCollection();
            services.AddMediatR();
            var sp = services.BuildServiceProvider();

            using (var scope = sp.CreateScope())
            {
                var mediator = scope.ServiceProvider.GetRequiredService<IMediator>();

                var foo = new Foo();
                await mediator.Publish(foo);
            }

            Console.WriteLine("Done");
            Console.ReadKey();
        }
    }

    public class Foo : INotification { }

    public class FooHandler : INotificationHandler<Foo>
    {
        public Task Handle(Foo notification, CancellationToken cancellationToken)
        {
            Console.WriteLine($"Handler: {GetType().Name}");
            return Task.CompletedTask;
        }
    }

    public class GenericHandler<TNotification> : INotificationHandler<TNotification>
        where TNotification : INotification
    {
        public Task Handle(TNotification notification, CancellationToken cancellationToken)
        {
            Console.WriteLine($"Hander: {GetType().Name} where TNotification : {GetType().GetGenericArguments()[0].Name}");
            return Task.CompletedTask;
        }
    }
}

where does ServiceCollection object come from?