Render custom html below a custom form
You have to use the power of render arrays.

For your example, I would use something like that :

$form_class = 'Drupal\xyz\Forms\XyzActionsForm';
$form = [
  'form' =&gt; $this-&gt;formBuilder-&gt;getForm($form_class, $parameter),
  'custom' =&gt; [
    '#type' =&gt; 'container',
    'inside' =&gt; [
      '#markup' =&gt; 'custom html', // Or use the power of render with html_tag type
    ],
  ],
];

return $form;


Then you can manage cache and other stuff on your render array and build your response with it.