Apple One Premier & iCloud storage
If family members have iCloud storage plans, you sign up for Apple One Premier, do the photos in iCloud storage merge with others in your family or do they continue to stay separate by person.
iPhone XS
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The following is applicable to iCloud storage sharing, whether part of Apple One or done separately. See below from Share an iCloud storage plan with your family - Apple Support
When you share a storage plan, your photos and documents stay private and everyone keeps using their own accounts with iCloud—just like when you have your own storage plan. You simply share the space available in the iCloud storage plan with your family members, so you only have one plan to manage.
If you want to share selected photos, you can do that by using the shared photo album. See below from Family Sharing - Apple
When you set up your family, a shared album is created automatically in the Photos app on all family members’ devices. Everyone can add photos, videos, and comments to the album whenever they like and get notified when something new is added.