Where to set the default "to" email address

It seems that this is the Site e-mail adres in the 2.x version

You can find this under /admin/config/system/site-information/ -> E-mail.

 $default_from_email = \Drupal::config('system.site')->get('mail');

webform_email_reply/src/Form/WebformEmailReplyForm.php

That's true for the From: value (who's sending the email), but I think the original request was about the To: value (who's receiving it)—and I actually just came looking for that this week as well. Ideally, I would think you'd be able to either enter a token that would fill in the value from the Webform submission, or you'd get to choose on a per-form basis which Email field from the form gets used.