If you’ve ever made a nib-based app before then you always had a MainWindow.xib file. This nib contained the top-level UIWindow object, a reference to the App Delegate, and one or more view controllers. When you put your app’s UI in a storyboard, however, MainWindow.xib is no longer used. So how does the storyboard get loaded by the app?
Let’s take a peek at the application delegate. Open up AppDelegate.h and you’ll see it looks like this