How to get product variations inside webforms?
You should be able to use the hook_form_alter() to achieve this in a custom module. As long as there is a relationship between the product and its variations you can use the hook_form_alter to add and remove variations from the form. You could also use the module controller to generate a custom twig template that renders content pull from the DB.

hook form alter -
 https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Form%21form.api.php/function/hook_form_alter/8.4.x

custom twig template -
https://www.drupal.org/docs/8/theming/twig/create-custom-twig-templates-from-custom-module

custom webform twig template - 
How to access webform elements in webform.html.twig