Extract email addresses from composite field type using Computed Twig
I'll provide 2 links as an answer, but it has really been answered by others.


Write custom webform handler:


How to create a custom Webform handler to submit to a SOAP API


Use hook_form_alter() to add a custom submit handler.


Add a custom submission handler to a form

Drupal 8 version of hook_form_alter(): https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Form%21form.api.php/function/hook_form_alter/8.8.x