Some years ago, I scanned all 6000 photos from my late grandmother’s extensive album.
They now exist in my Photos library on iOS and iPadOS.
I want to share that album with my mother and other relatives.
My mother has an iPad Mini and an iPhone.
How do I best share that album with her from the Photos app?
Also, can she update metadata like adding names to faces? If so will that metadata be added back to the album on my device?
Thanks 
You could create a Shared Photo Album send an invitation to your mother. Shared Photo Albums don’t count against your iCloud storage.
Unfortunately I don’t think it’s possible to add metadata using the i(Pad)OS version of Photos.
Edit 1: Apparently it is possible to identify faces on iPadOS. I don’t know if it’s possible with a shared album. though.
Edit 2: Caveat emptor: I have no idea what will happen if you decide to create a shared album containing that many photos. The biggest ones I’ve created have contained only a few hundred photos.
As of iOS/iPadOS 14 you can add photo captions on device (finally!). Open a photo then swipe up and a Caption field appears. This is also possible on the Mac version from the Info window for the photo.
However, it does appear you have to do this from your library rather than the shared album, where there is instead a Comments entry. So your mother won’t be able to update the caption but could add comments which you will see (and be notified of).
Would all the photos need to be in iCloud as well? Is it possible to not have them on icloud, but have them on shared photo albums?
No. I don’t even have iCloud photos turned on but I can create & share albums.
Thanks! That’s really good to know. I was looking for a good way to share without storing in icloud.
To accomplish this, turn on only Shared Albums in AppleID->iCloud->Photos.
Just wanted to report back. It’s only possible to share max 5000 photos in a shared photo album in iCloud. So I shared two albums of 3000 photos each.
It works really well! I had my suspicions with albums Of so many photos. But The photos were shared fast and it just works. my mother out her can put comments to the photos of people and places and dates and then I can edit the original and photo in my album in iCloud.
Thanks!
I do have a single follow-up question, maybe @jec0047 Jim can help out. How doI share these albums with people who are NOT on Apple devices of any sort… But on Android and other inferior platforms…?
When you create the album there’s an option to make it “public” (I believe that’s the term). That will give you a URL that you can share. You can make the album editable to other Apple users, which means they can add photos to the album, but non-Apple users can only view the album.
Edit: The last time I tried this, I was not impressed with the quality of the public images. However, I didn’t put any effort into it other than verifying the URL actually worked. I did get some favorable feedback from an Android user so maybe the quality of a downloaded image is OK.