Your backups are stored in the directory backups on your web space. It is located one level above the directory where your web space files are stored. To access this directory you need to login via sftp. You cannot use File Manager for this.
The easiest way to restore or download a backup is from Backup & Restore in the Control Panel.
Website Builder backups are stored on a separate server and not on your web space. You can only access them from inside Website Builder.