Need to buy more data,but how

Here is a difference that can cause some confusion:

… What's the difference between device storage and iCloud storage? - Apple Support

Internal storage cannot be increased. Only can be contents off-loaded, or contents deleted.

Prices:

… iCloud+ plans and pricing - Apple Support

How to upgrade iCloud storage:

… Upgrade to iCloud+ - Apple Support

By more "data" are you referring to your Apple ID's iCloud storage?