When your users attach a file into an attachment field, it is not stored in the Cassandra database with the rest of their submission. It is, instead, stored in whatever filehub structure (Local, S3, Azure, Sharepoint, etc) you have set up and connected to that Space. A reference to that file is stored in the attachment field.
The data contains the content type, the name of the file, and a link to the file in filehub. However, this link is not the ideal link for most use in downloading. There is a specific API endpoint for downloading attachments from a submission. This endpoint will give you a link that is valid for downloading the file for 5 seconds.
This time of expiring link is what is used when the user clicks on an attachment in the UI and in, for example, the clone attachment handler when the attachment is retrieved to copy.