환경

1. 각 탭에 표시할 ViewController 준비

import UIKit

class FirstViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        view.backgroundColor = .red
    }
}

2. ViewController를 UITabBarController로 설정

import UIKit

class MainTabController: UITabBarController {

    override func viewDidLoad() {
        super.viewDidLoad()

        configureViewControllers()
    }

    func configureViewControllers() {

        let first = FirstViewController()
        first.tabBarItem = UITabBarItem(title: "Home", image: UIImage(systemName: "house"), selectedImage: nil)

        let second = SecondViewController()
        second.tabBarItem = UITabBarItem(title: "Chat", image: UIImage(systemName: "message"), selectedImage: nil)

        let third = ThirdViewController()
        third.tabBarItem = UITabBarItem(title: "Book", image: UIImage(systemName: "book"), selectedImage: nil)

        let fourth = FourthViewController()
        fourth.tabBarItem = UITabBarItem(title: "Person", image: UIImage(systemName: "person"), selectedImage: nil)

        viewControllers = [first, second, third, fourth]
    }
}

Storyboard를 사용하지 않는 설정으로 변경

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 = MainTabController()
        window?.makeKeyAndVisible()
    }