How to render a weight element?
The select element is built in Weight::processWeight():

  /**
   * Expands a weight element into a select element.
   */
  public static function processWeight(&amp;$element, FormStateInterface $form_state, &amp;$complete_form) {
    $element['#is_weight'] = TRUE;

    $element_info_manager = \Drupal::service('element_info');
    // If the number of options is small enough, use a select field.
    $max_elements = \Drupal::config('system.site')-&gt;get('weight_select_max');
    if ($element['#delta'] &lt;= $max_elements) {
      $element['#type'] = 'select';
      $weights = [];
      for ($n = (-1 * $element['#delta']); $n &lt;= $element['#delta']; $n++) {
        $weights[$n] = $n;
      }
      $element['#options'] = $weights;
      $element += $element_info_manager-&gt;getInfo('select');
    }
    // Otherwise, use a text field.
    else {
      $element['#type'] = 'number';
      // Use a field big enough to fit most weights.
      $element['#size'] = 10;
      $element += $element_info_manager-&gt;getInfo('number');
    }

    return $element;
  }


Which is only called in form building, not in normal rendering.