I want to buy storage for iPhone

You can not increase the on phone storage.

iCloud is online backup storage that doesn’t increase your on phone storage.

You can’t add storage space to any phone. If you need more space, you will need to delete something.

I want to buy storage for my iPhone

Hello ~ If you want to buy iCloud storage:

iCloud - 50GB, 200GB, or 2TB plans

Take a look here for information:

What's the difference between device storage and iCloud ...

~Katana-San~