Currently, the wording for a project to project copy for Jira cloud using the new data copy method in admin.atlassian.com states the following:

We’ll copy issues, attachments, and configurations related to the projects you select. View the full list of what gets copied

It would be helpful for newer users who are unfamiliar with this process to include a warning (perhaps in bolded or red text) that states that the destination project would be overwritten with the copy data, as this will help avoid issues where a backup was not created or any further issues.