buildGroupSubmit example

    $form_state->unsetValue('expose_button');
    $form_state->unsetValue('group_button');

    if (!$this->isAGroup()) {
      $this->operatorSubmit($form, $form_state);
      $this->valueSubmit($form, $form_state);
    }
    if (!empty($this->options['exposed'])) {
      $this->submitExposeForm($form, $form_state);
    }
    if ($this->isAGroup()) {
      $this->buildGroupSubmit($form, $form_state);
    }
  }

  /** * Shortcut to display the operator form. */
  public function showOperatorForm(&$form, FormStateInterface $form_state) {
    $this->operatorForm($form, $form_state);
    $form['operator']['#prefix'] = '<div class="views-group-box views-left-30">';
    $form['operator']['#suffix'] = '</div>';
  }