How can I add a custom form to the output of a controller class?
I think you want to return a render array, instead of markup. Because the from needs to stay as a render array, your best bet it to bring your content content to the same format instead of the other way. In your example:

public function processSingle($nid) {
  $build = [];
  $build['heading'] = [
    '#type' =&gt; 'markup',
    '#markup' =&gt; '&lt;div class="process-heading"&gt;' . $this-&gt;t('My custom Form Is below but there will be alot more html than just the form. So this is just an example.') . '&lt;/div&gt;',
  ];
  $build['form'] = $this-&gt;formBuilder()-&gt;getForm('Drupal\my_module\Form\MyFormClassGoesHere', $nid);
  return $build;
}