SceneDelegate.swift

class SceneDelegate {
   var navigationController: UINavigationController!
 ...
    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
      guard let sceneWindow = (scene as? UIWindowScene) else { return }
      window = UIWindow(windowScene: sceneWindow)
      window?.makeKeyAndVisible()
      navigationController = UINavigationController(rootViewController: makeRootViewController())
      window?.rootViewController = navigationController
    }
  
  func makeRootViewController() -> UIViewController {
    let viewControllerA = ...
    viewControllerA.onButtonTappedObserve = {
      let viewControllerB = storyboard!.instantiateViewController(identifier: "ViewControllerB")
      navigationController.pushViewController(viewControllerB, animated: true)
    }
    return viewController A
  }
  ...
}