function mymodule_entity_type_build(array &$entity_types) {
  $entity_types['node']->setFormClass('my_custom_form_display_mode_id', 'DrupalnodeNodeForm');
}

$user = Drupal::entityTypeManager()
->getStorage('user')
->load(1);
$formObject = Drupal::entityTypeManager()
  ->getFormObject('user', 'my_custom_form_display_mode_id')
  ->setEntity($user);
$form = Drupal::formBuilder()->getForm($formObject);