Form in custom module using Controller: The HTML tag form isn't present in Twig
This is not working, you can't use a form template in a controller. You have to assign the template to $form['#theme'] when building the form or in a form alter hook. Then it gets used by formBuilder()-&gt;getForm() automatically.
See for example seven_form_node_form_alter():
$form['#theme'] = ['node_edit_form'];

and the corresponding twig file for a two column layout:
core/themes/seven/templates/node-edit-form.html.twig
{#
/**
 * @file
 * Theme override for a node edit form.
 *
 * Two column template for the node add/edit form.
 *
 * This template will be used when a node edit form specifies 'node_edit_form'
 * as its #theme callback.  Otherwise, by default, node add/edit forms will be
 * themed by form.html.twig.
 *
 * Available variables:
 * - form: The node add/edit form.
 *
 * @see seven_form_node_form_alter()
 */
#}
&lt;div class=&quot;layout-node-form clearfix&quot;&gt;
  &lt;div class=&quot;layout-region layout-region-node-main&quot;&gt;
    {{ form|without('advanced', 'footer', 'actions') }}
  &lt;/div&gt;
  &lt;div class=&quot;layout-region layout-region-node-secondary&quot;&gt;
    {{ form.advanced }}
  &lt;/div&gt;
  &lt;div class=&quot;layout-region layout-region-node-footer&quot;&gt;
    &lt;div class=&quot;layout-region-node-footer__content&quot;&gt;
      {{ form.footer }}
      {{ form.actions }}
    &lt;/div&gt;
  &lt;/div&gt;
&lt;/div&gt;


Form modes
A different approach would be to configure a form mode in UI containing only the field you want and then use it in your controller:
    $formObject = \Drupal::entityTypeManager()
        -&gt;getFormObject('user', 'custom_form_mode')
        -&gt;setEntity($user);

See How do I load a form with a specific form display (form mode)?