icloud storage separation

Depends on what you mean by "data of each device".

Backups for instance are separated by device and are identified by device name.

Other things such as files, contacts, photos etc.. are not separate and will be mixed. This should not be a problem if only one person is using the devices. It's all the same person's data and is normal to want it to be accessible on all the person's devices.

But if multiple people are using these devices, then each person should have their own Apple ID to keep their files and data separate. Even if using Family sharing and sharing iCloud storage.

There isn't a way to do that using the same Apple ID. Each device will sync to iCloud, the data of the devices will be combined, and downloaded to the other devices.

If you want separate storage, you need to use 6 Apple IDs for iCloud syncing.