Few questions are immediately deleted once they are closed. For example, they could be deleted when the user keep asking exactly the same question more than once, and every time the question is closed because it is not constructive, or not a real question.
In the other cases, when the question didn't get any answer, they are left there as the user is allowed to edit them to make them a better suit for the site. Editing the tags used from a question that is close is perfectly fine for two reasons:
- Editing the tag is done to educate the user about which tags should be used
- As a closed question is visible to other users too, removing wrong tags educates other users as well
Since on Drupal Answers there aren't users who vote to delete questions, there are questions that are kept open for long time after they are closed. If those questions use wrong tags, they should be edited in the same way open questions are edited.
Generally speaking, closing a question doesn't mean "this question will be surely deleted."
Duplicate questions are generally not deleted, if not in the case the question uses the exactly same words used by another question, or in the case I used as example at the beginning of this answer. Duplicate question are used as placeholders to redirect the users to the duplicated answer. Anonymous users don't see the duplicate questions; they are automatically redirected to the question that has been duplicated. I don't recall if it happens also for very low reputation users, but it is probable they are automatically redirected to the duplicated question.