Add separate div after a form
There might be hundreds of ways to add this markup in Drupal. The two simplest that came to my mind are:

1) Add a suffix to the form tag in form build or form alter:

$form['#suffix'] = '&lt;div&gt;Suffix to form&lt;/div&gt;';


2) or add the markup in a form.html.twig below the form tag:

{#
/**
 * @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()
 *
 * @ingroup themeable
 */
#}
&lt;form{{ attributes }}&gt;
  {{ children }}
&lt;/form&gt;

&lt;div&gt;
  A div container after the form
&lt;/div&gt;


For the second solution you might also need to add a theme name suggestion.