I am thinking of storing a reasonable amount of photos on my iCloud Drive. What I was wondering is when viewing these photos on an iPhone/iPad (iOS device), do they permanently download to the device or are they temporarily downloaded? I am just concerned about the storage space on my devices filling up with photos I'm only viewing.
I think you mean you're thinking of storing your photos at iCloud Photo Library, and not at iCloud Drive, because they're both different.
The photos are stored in the cloud and also on your device, however, to save space on your device, you will need to enable to 'Optimize iPhone Storage' feature on the phone.
Files in iCloud Drive sync with your device like all other iCloud content.
No space savings for photos doing it this way.
It's not like Dropbox where the files are "on-demand"
Test this yourself. Check the space being used by your iCloud Drive. Settings > General > Storage and iCloud Usage.
Drop 100MB of photos to your iCloud Drive. Check the storage again after it syncs. You should see the storage go up by 100MB.
iCloud drive download to iPhone/iPad
