Adding CSS or JS to admin pages is easy. Just create a new library, create CSS and JS files, and then attach the library in the hook_page_attachments() hook.

MY_MODULE.libraries.yml

admin_styling:
  version: VERSION
  css:
    theme:
      css/admin-styling.css: {}
  js:
    js/admin-script.js: {}

MY_MODULE.module

/**
 * Implements hook_page_attachments().
 */
function MY_MODULE_page_attachments(array &$attachments) {
  if (\Drupal::service('router.admin_context')->isAdminRoute()) {
    $attachments['#attached']['library'][] = 'MY_MODULE/admin_styling';
  }
}

If you want to hide the generator metatag that exposes information about your Drupal version then you can use the hook_page_attachments_alter() hook. This applies only for versions 8 and 9!

/**
 * Implements hook_page_attachments_alter().
 */
function MY_MODULE_page_attachments_alter(array &$attachments) {
  foreach ($attachments['#attached']['html_head'] as $key => $attachment) {
    if (isset($attachment[1]) && $attachment[1] == 'system_meta_generator') {
      unset($attachments['#attached']['html_head'][$key]);
    }
  }
}

To save the API token field without submitting the form, you can add a simple ajax callback function. In this example, we are saving the field value on the keyup event, but you can also use the change event or if you have an autocomplete field then you probably want to use the autocompleteclose event.

use Drupal\Core\Ajax\AjaxResponse;
use Drupal\Core\Form\FormStateInterface;

public function buildForm(array $form, FormStateInterface $form_state) {
  $form['api_token'] = [
    '#type' => 'textfield',
    '#title' => $this->t('API token'),
    '#ajax' => [
      'callback' => '::updateApiToken',
      'event' => 'keyup',
    ],
  ];

  return parent::buildForm($form, $form_state);
}

public function updateApiToken(array $form, FormStateInterface $form_state) {
  $response = new AjaxResponse();
  $this->config('MY_MODULE.settings')
    ->set('api_token', $form_state->getValue('api_token'))
    ->save();
  return $response;
}

Let's say that you have a multivalue reference field named field_tags in a node type. To remove a specific item from that field you can do something like this:

$node_id = 152;
$tag_to_remove = 56;
$entity_storage = \Drupal::entityTypeManager()->getStorage('node');
$entity = $entity_storage->load($node_id);

foreach ($entity->get('field_tags') as $delta => $field_item) {
  if ($field_item->target_id == $tag_to_remove) {
    $entity->get('field_tags')->removeItem($delta);
    $entity->save();
    break;
  }
}

Adding a redirect to hook_user_login() is a common mistake. You should never add the redirect there because redirecting users in that hook would stop other hook_user_login() implementations from being invoked. To properly redirect users to some page after they log in, you should add a custom submit handler to the user login form.

use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Url;

/**
 * Implements hook_form_FORM_ID_alter().
 */
function MY_MODULE_form_user_login_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  $form['#submit'][] = 'MY_MODULE_user_login_form_submit';
}

/**
 * Custom submit handler for the login form.
 */
function MY_MODULE_user_login_form_submit($form, FormStateInterface $form_state) {
  $url = Url::fromRoute('<front>');
  $form_state->setRedirectUrl($url);
}