I do not see the initial ViewController in appDelegate swift

I'm used to defining in the application appDelegate the viewController with which the application starts, for example:

 func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {

    //inicializar la aplicación
    window = UIWindow(frame: UIScreen.main.bounds)

    let mainVC = MainVC()
    let navigationController = UINavigationController(rootViewController: mainVC)

    navigationController.setNavigationBarHidden(true, animated: true)
    window?.rootViewController = navigationController
    window?.makeKeyAndVisible()
    return true
}

However, I have found projects (in cocoapods especially) that have nothing defined in the appDelegate and the application starts in a specific Controller, even though there is more than one in the same route.

What is the execution rule of the ViewController default? Is there something inside the controller that I am omitting and that allows to run as default controller?