Which document do I need to read on how to make an app?

Have you looked at AppDevelopment with Swift from Apple ?

Entry point is defined by @UIApplicationMain in AppDelegate
It contains the code to execute once app is launched:

Code Block 
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool { }

May have a look at this thread:
https://developer.apple.com/forums/thread/112690

Claude31, thank you for your reply.

However, I can see that Xcode 12 uses @main entry point and not @UIApplicationMain in AppDelegate.

Basically, what I'm saying is, as a newbie, I haven't a clue where to learn about app architecture. Is there someone who can point me to a good resource on Apple's Documentation. I don't think something like this should be hidden. It should be made obvious to anyone coming to Apple's Documentation site for the first time, don't you think?

Are you selecting SwiftUI when you create the app?

Have a look here for detailed explanation about main
https ://swiftrocks. com/entry-points-swift-uiapplicationmain-main

For more general, search in Archive
https://developer.apple.com/library/archive/navigation/

You'll find documents like this for Cocoa
https://developer.apple.com/library/archive/documentation/General/Conceptual/MOSXAppProgrammingGuide/Introduction/Introduction.html#//apple_ref/doc/uid/TP40010543

As stated, those types of docs are no more updated…
You may ask Apple support for further references.

Claude, thanks for your reply.

To answer your question, no I'm not using SwiftUI. I'm selecting Storyboard but I still see @main.

I think I found my answer in the Swift Language Reference but I'm still baffled by the app architecture. I guess I'll simply have to go ahead and work my way through "Start Developing An iOS App" and see where that leads me.

Thanks again Claude.