Set initial view controller when not working with storyboards (don't forget to remove Main.storyboard from Build Settings)

AppDelegate.swift

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
    // Override point for customization after application launch.
        
    let vc = ViewController()
    let window = UIWindow(frame: UIScreen.main.bounds)
    window.rootViewController = vc
    window.makeKeyAndVisible()
    self.window = window
        
    return true
}