Send an email when a Drupal 8 entity has been updated.

mailer.module

<?php 

use Drupal\Core\Entity\EntityInterface;

function HOOK_entity_update(EntityInterface $entity) {
 $host = 'local';
  if (array_key_exists('AH_SITE_ENVIRONMENT', $_ENV)) {
    $host = $_ENV['AH_SITE_ENVIRONMENT'];
  }
  elseif (array_key_exists('AH_SITE_ENVIRONMENT', $_SERVER)) {
    $host = $_SERVER['AH_SITE_ENVIRONMENT'];
  }

  // Check whether current environment is a dev environment based on $host.
  $is_dev = (in_array($host, ['local', 'dev', 'test']));
  
    $params = [];
  $vendor_email = \Drupal::config('mailer.settings')->get('admin_email');
  $user = User::load(\Drupal::currentUser()->id());
  $i = 0;

  if ($entity->getEntityTypeId() == 'node' && $entity->getType() == 'article' && $user->hasRole('user')) {
    $params[$i]['key'] = 'user-updates-article';
    $params[$i]['email_to'] = $vendor_email;
    $params[$i]['template_name'] = 'partner-updates-city.html.twig';
    $params[$i]['subject'] = 'An Article Update Has Been Made - ' . $entity->get('title')->value;
    $params[$i]['reply_email'] = \Drupal::currentUser()->getEmail();
    $params[$i]['copy_vendor_email'] = FALSE;
    $params[$i]['vars'] = [
      'title' => $entity->get('title')->value,
      'nid' => $entity->id(),
      'url' => $entity->toUrl()->toString(),
      'user_email' => $user->get('mail')->value,
      'is_dev' => $is_dev,
    ];
    $i++;
  }
}