I want storage

You can't add more storage to your iPhone after purchase, but you can upgrade your iCloud storage. Doing so might allow you to free up some space on your phone, but they aren't the same.

These articles might help:

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

Optimize storage on your iPhone and iPad - Apple Support

Manage your photo and video storage - Apple Support

What you bought is iCloud storage and not iPhone storage.