Are you wondering what is the application entry point? How the OS knows where to find an entry point? What @UIApplicationMain attribute does? What's hidden behind the UIApplicationMain method? If you want to know the answer to those questions, you should probably read this article.
Have you ever faced the situation, when you did not know why the code from external framework works this way and not the other? I'll try to show various methods how to look inside the framework binary to understand how it works.