NavigationWithEncapsulation.swift

import UIKit

class SceneDelegate: UIResponder, UIWindowSceneDelegate {

    var window: UIWindow?

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

class ViewController: UIViewController {
    
    @IBAction func buyTapped(_ sender: Any) {
        navigationController?.pushViewController(BuyViewController.instantiate(), animated: true)
    }

    @IBAction func createAccount(_ sender: Any) {
        navigationController?.pushViewController(CreateAccountViewController.instantiate(), animated: true)
    }
}

class BuyViewController: UIViewController {}
class CreateAccountViewController: UIViewController {}

extension UIStoryboard {
    static var main: UIStoryboard {
        .init(name: "Main", bundle: Bundle.main)
    }
}

extension UIViewController {
    /// Requires the storyboard id to match the class name. 
    static func instantiate(storyboard: UIStoryboard = .main) -> Self {
        storyboard.instantiateViewController(withIdentifier:  NSStringFromClass(self).components(separatedBy: ".")[1]) as! Self
    }
}