To check this we need to use router.admin_context service

if (\Drupal::service('router.admin_context')->isAdminRoute()) {
   // Do something here.
}

We also can check a specific route

$route_provider = \Drupal::service('router.route_provider');
$route = $route_provider->getRouteByName('user.page');
if (\Drupal::service('router.admin_context')->isAdminRoute($route)) {
   // Do something here.
}

In some cases we want to load a configuration file from optional / install folders. For example to compare it with active config and apply new changes.
To load a configuration from install folder user the next code snippet.

use Drupal\Core\Config\ExtensionInstallStorage;

$field_storage = 'field.storage.group_content.grequest_status';
$install_storage = new ExtensionInstallStorage(
      \Drupal::service('config.storage'),
      InstallStorage::CONFIG_INSTALL_DIRECTORY,
      InstallStorage::DEFAULT_COLLECTION,
      TRUE,
      NULL
);

From optional folder

use Drupal\Core\Config\ExtensionInstallStorage;

$name = 'views.view.group_pending_members';

$optional_storage = new ExtensionInstallStorage(
      \Drupal::service('config.storage'),
      InstallStorage::CONFIG_OPTIONAL_DIRECTORY,
      InstallStorage::DEFAULT_COLLECTION,
      TRUE,
      NULL
);

and schema configuration

use Drupal\Core\Config\ExtensionInstallStorage;

$name = 'views.view.group_pending_members';

$optional_storage = new ExtensionInstallStorage(
      \Drupal::service('config.storage'),
      InstallStorage::CONFIG_SCHEMA_DIRECTORY,
      InstallStorage::DEFAULT_COLLECTION,
      TRUE,
      NULL
);

For this we need implement user_login hook and then we check the last access field.

use Drupal\user\Entity\User;

/**
 * Implements hook_user_login().
 */
function mymodule_user_login(User $user) {
  if (empty($user->getLastAccessedTime())) {
    // Do something here.
  }
}

Let's imagine we want to send a user to custom route after login in our example to user edit form. How do we do it ?

We need to implement user_login hook

use Drupal\Core\Url;
use Symfony\Component\HttpFoundation\RedirectResponse;
use Drupal\user\Entity\User;

/**
 * Implements hook_user_login().
 */
function mymodule_user_login(User $user) {
    $url = Url::fromRoute('entity.user.edit_form', [
       'user' => $user->id(),
    ]);
    $redirect = new RedirectResponse($url->toString());
    $redirect->send();
}

We need to user service "messenger" service, which will store the message in the session and fetch during the next request.

Get instance of messenger service

# 1. Standard way to get messenger service, in case you want to pass it using dependency injection.
$messenger = \Drupal::service("messenger");

# 2. Fast way to use helper function.

$messenger = \Drupal::messenger();

Also in the form object or controller we don't have include it using dependency injection. We can us directly 

$messenger = $this->messenger();

Methods

1. Add message. Allows to send a message of any type, the second parameter accepts the type

$meesenger->addMessage(t('Your message.'), MessengerInterface::TYPE_WARNING);

But in most of the cases it is better to use directly specific method

2. Add status

$messenger->addStatus(t('Your message'));

3. Add warning

$messenger->addWarning(t('Your message'));

4. Add error

$messenger->addError(t('Your message'));