Creating a webform modal is quite simple and straight forward in Drupal 8. We only need the Webform module and some classes for the modal opening button: Download and install https://www.drupal.org/project/webform Create a webform
Some days ago we had the requirement to display some entityforms and other Drupal Core forms like the login in modal windows. The second requirement is easy to solve. Many modules like colorbox.module, lightbox.module and others solve this already.