How do I know what was backed up on hyperbackup? I have an hbk file but I cannot open it or see its size.
https://kb.synology.com/en-us/DSM/tutorial/How_to_restore_settings_apps_files_permissions_by_Hyper_Backup
Restoring files from previously backups.
https://kb.synology.com/en-us/DSM/help/HyperBackup/restore?version=7
Howto retrieve files already backed up by Hpyerbackup.
https://kb.synology.com/en-us/DSM/tutorial/How_to_retrieve_backup_files_with_Hyper_Backup_Explorer
So actually use hyper backup, hyper backup explorer on a supported client pc/laptop device or hyper vault (the latter if you use a 2nd synology as backup target) and work from there.
And a backup is only as good as the last succesful restore one is able to perform with it. So not unless tested and verified, a backup in and by itself is worthless.