buildInputElement example

    // the user.     $form['status_messages'] = [
      '#type' => 'status_messages',
    ];

    $form['#attributes']['class'] = [
      'js-media-library-add-form',
    ];

    $added_media = $this->getAddedMediaItems($form_state);
    if (empty($added_media)) {
      $form = $this->buildInputElement($form, $form_state);
    }
    else {
      $form['#attributes']['data-input'] = 'true';

      // This deserves to be themeable, but it doesn't need to be its own "real"       // template.       $form['description'] = [
        '#type' => 'inline_template',
        '#template' => '<p>{{ text }}</p>',
        '#context' => [
          'text' => $this->formatPlural(count($added_media), 'The media item has been created but has not yet been saved. Fill in any required fields and save to add it to the media library.', 'The media items have been created but have not yet been saved. Fill in any required fields and save to add them to the media library.'),
        ],