My purchased storage doesn’t appear in my iPhone

You cannot purchase storage for the phone. You need to take steps to utilise the iCloud storage you purchased

Managing iCloud storage and what is the difference between device storage and iCloud storage?

You can buy iCloud storage space and move some of your data there. 

Manage your iCloud storage – Apple Support (UK)

and

What’s the difference between device storage and iCloud storage? – Apple Support (UK)