Drupal: Creating pages for a premium account programmatically

drupalbook.info.yml

name: DrupalBook
description: Custom module for learning Drupal
type: module
core: 8.x
core_version_requirement: ^8 || ^9
package: DrupalBook

drupalbook.permissions.yml

access premium pages:
  title: 'Access Premium pages'
  description: 'A custom permission for your pages.'
  restrict access: TRUE

drupalbook.routing.yml

drupalbook.private_content:
   path: '/private-page'
   defaults:
     _controller: '\Drupal\drupalbook\Controller\FirstPageController::privateContent'
     _title: 'Private content'
   requirements:
     _permission: 'access premium pages'

FirstPageController.php

<?php

/**
 * @file /modules/custom/drupalbook/src/Controller/FirstPageController.php
 */

namespace Drupal\drupalbook\Controller;
 
/**
 * Provides route responses for the DrupalBook module.
 */
class FirstPageController {
 
  /**
   * Returns a private page.
   *
   * @return array
   *   A simple renderable array.
   */
  public function privateContent() {
    $element = array(
      '#markup' => 'Private content',
    );
    return $element;
  }
 
}