1. SceneDelegate 내용을 다음과 같이 변경

class SceneDelegate: UIResponder, UIWindowSceneDelegate {
    var window: UIWindow?

    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        guard let scene = (scene as? UIWindowScene) else { return }
        window = UIWindow(windowScene: scene)
        window?.rootViewController = ViewController()
        window?.makeKeyAndVisible()
    }

    /* 생략 */
}

2. ViewController를 다음과 같이 구성

class ViewController: UITabBarController {
    override func viewDidLoad() {
        super.viewDidLoad()
        let timelineViewController = UINavigationController(rootViewController: TimelineViewController())
        timelineViewController.tabBarItem = UITabBarItem(
	        title: "",
	        image: UIImage(systemName: "house"),
			selectedImage: UIImage(systemName: "house.fill")
		)

        let tabBars = [timelineViewController]    
        viewControllers = tabBars
    }
}

+ 다른 화면을 추가로 개발하게 되면 뷰 컨트롤러 객체 생성 후 tabBars에 추가