Drupal - Redirect after form submission
In your "submitForm" method write below code
$form_state->setRedirect('machine_name');
return;
where machine_name
is the machine name mentioned in the routing file.
I hope this helps .. :)
If you want to set at buildform you need to use some routing path like.
use Drupal\Core\Url;
$url = Url::fromRoute('route.path');
$form_state->setRedirectUrl($url);