See the following 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.
and see the following from What happens to your iCloud storage plan when you sign up for Apple One - Apple Support
If a family member subscribes to Apple One
If you belong to a Family Sharing group and a family member subscribes to an Apple One Family or Premier plan, you can choose to keep your current iCloud storage plan or use the shared iCloud storage in Apple One.
If your family needs additional iCloud storage, the family member who purchased Apple One can buy more iCloud storage. With both Apple One and an iCloud storage plan, your family can share up to 4TB of total iCloud storage.