Send form result back to twig
In submit you have two choices.

Rebuild

public function submitForm(array &amp;$form, FormStateInterface $form_state) {

  $form_state-&gt;setRebuild();
}


The form and the twig template the form is wrapped in will be rebuilt and you can display the result.

This is a great question from @AdrianCidAlmaguer to demonstrate how this works with a basic example how to display the sum of two numbers: How to show a string once you submit a form in a page reloading the page?

Redirect

public function submitForm(array &amp;$form, FormStateInterface $form_state) {

  $form_state-&gt;setRedirect($route_name, $route_parameters, $options);
}


You redirect to a new page and can display the result there.