Apple One Subscription-Storage

Family Members can choose to keep their own storage or share the one from the Family Plan of Apple One.

Wether they keep their own, or share the Apple One storage, each person still retains their own independent account, and files are not shared or visible between Family Members.

You get to use the space, but you don't see any of the other peoples files, nor can they see any of yours.

You can choose not to use the shared Family Storage and use your own instead. If you ever change your mind you can always go back and select to use the shared storage.

If you are the family organizer then you can choose not to share storage with other family members. It will only be shared if you set it up to be shared.

As a family member, if you choose to use the family shared storage it will replace your own iCloud storage subscription.

From the link provided by Limnos above:

If a family member is already paying for their own iCloud storage plan, they can choose to switch to your plan or keep their own and still remain part of the family. When they switch to the shared family plan, they get a refund for the remainder of their personal plan. They can't keep their current plan and use the shared family plan at the same time.

I would think it would be the same as discussed in this article where you can select it or not.

Share an iCloud storage plan with your family - Apple Support