Theme Ubercart product node and form in Drupal 8
This might work:

public function MYFUNCTION() {
  $myform = \Drupal::formBuilder()-&gt;getForm('Drupal\[mymodule]\Form\[myformclass]');
  // If you want modify the form:
  $myform['field']['#value'] = 'From my controller';

  $build = [
    '#theme' =&gt; 'mytemplate'
    '#form' =&gt; $myform,
  ];
}


In twig:

{{ form }}