func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
guard let windowScene = (scene as? UIWindowScene) else { return }
// 단일 뷰를 보여주는 전통적인 예시 코드
window = UIWindow(frame: windowScene.coordinateSpace.bounds)
window?.windowScene = windowScene
// 뿌리가 되는 ViewController -> 시작점을 정해준다.
window?.rootViewController = ViewController()
window?.makeKeyAndVisible()
}

window 변수의 의미(?)
/* SceneDelegate.swift */
let navigationViewController = UINavigationController(rootViewController: FirstViewController())
window = UIWindow(frame: windowScene.coordinateSpace.bounds)
window?.windowScene = windowScene
window?.rootViewController = navigationViewController
window?.makeKeyAndVisible()
/* FirstViewController.swift */
let detailViewContoller = DetailViewController()
navigationController?.pushViewController(detailViewContoller, animated: true)

let tabBarController = UITabBarController()
let firstViewController = UINavigationController(rootViewController: FirstViewController())
firstViewController.tabBarItem = UITabBarItem(title: "First", image: UIImage(systemName: "pencil.circle"), tag: 0)
let secondViewController = SecondViewController()
secondViewController.tabBarItem = UITabBarItem(title: "Second", image: UIImage(systemName: "eraser.fill"), tag: 0)
tabBarController.viewControllers = [firstViewController, secondViewController]
window = UIWindow(frame: windowScene.coordinateSpace.bounds)
window?.windowScene = windowScene
window?.rootViewController = tabBarController
window?.makeKeyAndVisible()
유익한 정보를 제공해주셔서 감사합니다.