Store uploaded files in different directory?

Where does Frappe store uploaded files for the sites? I currently have ERPNext installed on a VM that has NVMe storage, but I’d like to mount some HDD storage (cheaper per GB) to store files (as in backups and the files that users upload).

I see that the sites directory has

/sites/domain.com/public/files
/sites/domain.com/private/files

Where are the files that users upload stored? What’s the difference between private and public directories?