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If you share an iCloud Storage plan, your data stays where it is under your own Apple ID. Only the billing arrangement changes. See the following from Share an iCloud storage plan with your family - Apple Support

When you share your iCloud storage, any family members using the free 5GB plan are added to your family plan automatically.

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.

They will still be in the same place they always were and accessed via your Apple ID as they always were. The only thing that changes is how it is billed.