How can I alter the login form in the login block but not in the login page?

How should I use hook_form_alter() to modify the user login block but not the user login form page? Everything I've tried either modifies both entities or doesn't work, not even using the following hooks.

function mymodule_form_user_login_block_form_alter(&$form, &$form_state, $form_id) {
  // …
}

function mymodule_form_alter(&$form, &$form_state, $form_id) {
  if ($form_id == 'user_login_block') {
    // …
  }
}

It really depends on what you're trying to accomplish. The user login form has a form ID of user_login_form. That form, with that form ID, may be shown either in the user login block or on the /user page. It is not show twice on the same page - if you're on /user then the block will not be shown. To me, if you want to change that login form, you definitely should be changing it in both places that it appears. For that case, use hook_form_FORM_ID_alter() which in this case is hook_form_user_login_form_alter().

If you simply want to style your block differently, you can use CSS and target the #block-userlogin element or more specifically the #block-userlogin #user-login-form element.

Or, you can alter the block plugin using hook_block_alter().

Or, you can create your own custom block to show your own custom login form.

Or ...

Again, it really depends on what you're trying to do by altering just the block.

The form used in the login form and in the login page is the one implemented by the Drupal\\user\\Form\\UserLoginForm class. This means that the form ID to check in hook_form_alter() or hook_form_FORM_ID_alter() is the same in both the cases.

Implementing hook_block_view_BASE_BLOCK_ID_alter(), it's possible to alter the rendering output for a block, including the login block.

function mymodule_block_view_user_login_block_alter(array &$build, \Drupal\Core\Block\BlockPluginInterface $block) {
  // Alter the login form, which is stored in $build['user_login_form'].
  // This example changes the size of the name and password fields.
  $build['user_login_form']['name']['#size'] = 18;
  $build['user_login_form']['pass']['#size'] = 18;
}

The plugin ID for the login block is given in the annotation for the UserLoginBlock class. The content of the $build array is returned from UserLoginBlock::build().

  $form = \Drupal::formBuilder()->getForm('Drupal\\user\\Form\\UserLoginForm');
  unset($form['name']['#attributes']['autofocus']);

  unset($form['name']['#description']);
  unset($form['name']['#attributes']['aria-describedby']);
  unset($form['pass']['#description']);
  unset($form['pass']['#attributes']['aria-describedby']);
  $form['name']['#size'] = 15;
  $form['pass']['#size'] = 15;

  $placeholder = 'form_action_p_4r8ITd22yaUvXM6SzwrSe9rnQWe48hz9k1Sxto3pBvE';
  $form['#attached']['placeholders'][$placeholder] = [
    '#lazy_builder' => [
      '\\Drupal\\user\\Plugin\\Block\\UserLoginBlock::renderPlaceholderFormAction',
      [],
    ],
  ];
  $form['#action'] = $placeholder;

  $items = [];
  if (\Drupal::config('user.settings')->get('register') != UserInterface::REGISTER_ADMINISTRATORS_ONLY) {
    $items['create_account'] = [
      '#type' => 'link',
      '#title' => $this->t('Create new account'),
      '#url' => Url::fromRoute('user.register', [], [
        'attributes' => [
          'title' => $this->t('Create a new user account.'),
          'class' => [
            'create-account-link',
          ],
        ],
      ]),
    ];
  }
  $items['request_password'] = [
    '#type' => 'link',
    '#title' => $this->t('Reset your password'),
    '#url' => Url::fromRoute('user.pass', [], [
      'attributes' => [
        'title' => $this->t('Send password reset instructions via email.'),
        'class' => [
          'request-password-link',
        ],
      ],
    ]),
  ];
  return [
    'user_login_form' => $form,
    'user_links' => [
      '#theme' => 'item_list',
      '#items' => $items,
    ],
  ];