Extensions?
The Mac App Store currently lists only 5 extensions for photos and zero for videos...
when you use the search phrase 'photos extension'
Apple's articles show significantly more extensions available.
where can I download and install them?
the existence of extensions is only noted in the applications descriptions as far as I've found - I've only found five also - if you think that some app might have an extension then look at that app or contact it's authors - my guess is that the things you are seeing are not released yet since extensions are so new but the author can explain their product to you
LN