How to created the services in drupal 10

 Step 1. Create service file and add the class as service

Example : mymodule.services.yml

services:

  mymodule.custom_services:

    class: Drupal\mymodule\CustomService

    arguments: ['@current_user']

Step 2. Create the service Class

Example: src/CustomService.php

<?php

namespace Drupal\mymodule;

use Drupal\Core\Session\AccountInterface;

/**

 * Class CustomService

 * @package Drupal\mymodule\Services

 */

class CustomService {

  protected $currentUser;

  /**

   * CustomService constructor.

   * @param AccountInterface $currentUser

   */

  public function __construct(AccountInterface $currentUser) {

    $this->currentUser = $currentUser;

  }

  /**

   * @return \Drupal\Component\Render\MarkupInterface|string

   */

  public function getData() {

    return $this->currentUser->getDisplayName();

  }

}

Step 4: Now you can call your service from other modules.

Example how to call service in hook__preprocess_node():

<?php

/**
 *  run cron
 */
function mymodule_preprocess_node(&$variables){

  $data = \Drupal::service('mymodule.custom_services')->getData();
  var_dump($data);
}

Step 2. Create the Controller Class
Example: src/Controller/MyController.php
<?php
namespace Drupal\mymodule\Controller;
use Drupal\Core\Controller\ControllerBase;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Drupal\mymodule\CustomService;

class MyController extends ControllerBase
{
 /**
   * The data services.
   *
   * @var Drupal\mymodule\CustomService;
   */
 protected $info_data;

 public function __construct(CustomService $info_data)
  {
    $this->info_data = $info_data;
  }
 /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container)
  {
    return new static(
        $container->get('mymodule.custom_services')
    );
  }
/**
   * Builds the response.
   */
  public function information()
  {
    $info = $this->info_data->getData(); //First way using DependencyInjection 
    $data = \Drupal::service('mymodule.custom_services')->getData(); // Second way using service 
    $build['content'] = [
      '#markup' => 'hi this is test',
    ];
    return $build;
  }
}
module.routing.yml
module.test:
  path: '/test'
  defaults:
    _title: 'Example'
    _controller: '\Drupal\module\Controller\MyController::information'
  requirements:
    _permission: 'access content'

}