Create a custom Drupal module with route and controller

drupalbook.info.yml

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

drupalbook.routing.yml

drupalbook.first_page:
  path: '/first-page'
  defaults:
    _controller: '\Drupal\drupalbook\Controller\FirstPageController::content'
    _title: 'Hello World!'
  requirements:
    _permission: 'access content'

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 simple page.
   *
   * @return array
   *   A simple renderable array.
   */
  public function content() {
    $element = array(
      '#markup' => 'Hello World!',
    );
    return $element;
  }
 
}