I’m don’t understand what you mean by “custom scene delegate” or by “used”.

The TestingAppDelegate will create a UISceneConfiguration that is the first in the app’s Info.plist. But the storyboard for that scene configuration is not loaded (well, ViewController.viewDidLoad is not called when testing — the storyboard itself might be loaded). The TestingAppDelegate overides the scene configuration’s delegate so that TestingSceneDelegate is used instead of the delegate defined in the Info.plist. TestingSceneDelegate then will create its own window with TestingRootViewController.