ASP.NET Core, How Does Runtime Reason About Startup Class?

In Startup.cs I have a Startup class which exposes ConfigureServices() and Configure(). The Startup class is used in the Main() code path as such...

webBuilder.UseStartup<Startup>();

Many questions:

• How/why are Startup callback methods inferred if Startup does not implement an interface?
• Is the runtime using Reflection?
• Why isn't the generic type an interface?
• What if Startup class does not implement ConfigureServices() and Configure()?