at_theme-1.4.1/at_core/templates/form/form--search-block-form.html.twig

{#
/**
 * @file
 * Default theme implementation for a 'form' element.
 *
 * Available variables:
 * - attributes: A list of HTML attributes for the wrapper element.
 * - children: The child elements of the form.
 *
 * @see template_preprocess_form()
 */
#}
<form{{ attributes.addClass('search-block-form__form') }}>
  {{ children }}
</form>