Show a summary calculated from webform submitted values on the submission preview page only (not on form) and re-use in e-mail handler
It's not the way i'd prefer to do it, but as far as putting customization on the preview page, there is at least an alternate template available for that:
 FILE NAME SUGGESTIONS:
   * webform-submission-data--preview.html.twig
   * webform-submission-data--webform-10152--preview.html.twig
   * webform-submission-data--webform-10152.html.twig
   x webform-submission-data.html.twig

Rather than use the overall template above, we decided to do most of the work in a preprocess function for a key element on the webform.  This only affects the preview page, as it's about the HTML display not the form element.
function example_preprocess_webform_element_base_html__example_element__specific_instance(&amp;$vars) {
  $rows = example_summary($vars['webform_submission']);
  $vars['table_summary'] = [
   '#type' =&gt; 'table',  
   '#rows' =&gt; $rows,
  ];
}

We can take the {{ table_summary }} variable we added there and include it in the custom template for that element, webform-element-base-html--example-element--specific-instance.html.twig.