If you are using Moodle 3.5 or later, the way user deletions (particularly GDPR deletions) are processed is relatively complicated, because multiple considerations need to be made, such as identifying which data within the system specifically belongs to that user, what information needs to be retained for auditing purposes, and others.
In the GDPR FAQ documentation, (under 'See also'), there is a link to a response to an earlier question similar to yours, "Is there a way to completely delete users including all their data?", which is quite detailed, and I think will help answer your questions (you can find that response here). Some of the other responses in that forum discussion may also be helpful.
If you are using an older version of Moodle (3.4 or earlier), this pre-dates GDPR support and I believe users are simply flagged as deleted, so their accounts become inactive (but upgrading your site should make the improved deletion method available).