Basic example of creating iOS view and adding label with constraints programatically.

ViewController.swift

import UIKit

class ViewController: UIViewController {

  override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view.
  }
  
  override func loadView() {
    view = UIView()
    view.backgroundColor = .white
    
    let label = UILabel()
    label.translatesAutoresizingMaskIntoConstraints = false
    label.textAlignment = .center
    label.text = "hello, world"
    view.addSubview(label)
    
    NSLayoutConstraint.activate([
      label.centerXAnchor.constraint(equalTo: view.centerXAnchor),
      label.centerYAnchor.constraint(equalTo: view.centerYAnchor)
    ])
  }

}