Scroll view with the first element

ScrollViewController.swift

import UIKit

class ScrollViewController: UIViewController {

    let scrollView: UIScrollView = {
        let scrollView = UIScrollView()
        scrollView.translatesAutoresizingMaskIntoConstraints = false
        scrollView.backgroundColor = .lightGray
        return scrollView
    }()
    
    let containerView: UIView = {
           let view = UIView()
           view.translatesAutoresizingMaskIntoConstraints = false
           view.backgroundColor = .white
           return view
       }()
    
    let redButton: UIButton = {
        let btn = UIButton()
        btn.translatesAutoresizingMaskIntoConstraints = false
        btn.backgroundColor = .red
        return btn
    }()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .white
        addScrollView()
        addContainerView()
        addRedButton()
    }

    private func addScrollView() {
        view.addSubview(scrollView)
        scrollView.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
        scrollView.trailingAnchor.constraint(equalTo: view.trailingAnchor).isActive = true
        scrollView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor).isActive = true
        scrollView.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
    }
    
    private func addContainerView() {
        scrollView.addSubview(containerView)
        containerView.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
        containerView.trailingAnchor.constraint(equalTo: view.trailingAnchor).isActive = true
        containerView.topAnchor.constraint(equalTo: scrollView.topAnchor).isActive = true
        containerView.bottomAnchor.constraint(equalTo: scrollView.bottomAnchor).isActive = true
        
    }
    
    private func addRedButton() {
        containerView.addSubview(redButton)
        redButton.topAnchor.constraint(equalTo: containerView.topAnchor).isActive = true
        redButton.leadingAnchor.constraint(equalTo: containerView.leadingAnchor, constant: 10).isActive = true
        redButton.trailingAnchor.constraint(equalTo: containerView.trailingAnchor, constant: -10).isActive = true
        redButton.heightAnchor.constraint(equalToConstant: 200).isActive = true
        
        // IMPORTANT 🚨
        scrollView.bottomAnchor.constraint(equalTo: redButton.bottomAnchor).isActive = true

    }

}