I use iCloud Photo Library on my iPhone to sync my photos and videos with all my Apple devices. On my iMac, I have stored the originals of the photos and videos. If you wonder why, here is the answer: for backup.
It isnt’t that I don’t trust Apple. I think iCloud works reliably. But it is always safe to have a backup of your cloud storage, just for case.
Here is how I do the backup: as I mentioned the photos and videos originals are in the Photos Library on my iMac that has a 1 TB HDD. The external Thunderbolt SSD is the boot device but I’m not using it for storing the Photos Library that is synced with iCloud Photo Library, the 1 TB HDD takes that job (the HDD is still HFS+ formatted). So here lies the first copy of iCloud Photo Library.
The second copy of iCloud Photo Library is on the NAS. Carbon Copy Cloner copies the iCloud Photo Library from the HDD to a sparse bundle disk image (formatted to APFS) on the NAS. NAS is running on FreeBSD with ZFS.
And the third copy of iCloud Photo Library is on Backblaze’s B2. rclone syncs the sparse bundle disk image on the NAS with B2.