How do i buy more iphone storage for my device

You can not increase the amount of storage on any iPhone.

What amount of storage it came with is all it will ever have.

The iPhone storage was fixed when you bought your iPhone. The iPhone physical storage cannot be upgraded. You can get more iCloud virtual storage, but it is not the same thing and one cannot substitute for the other. See the 'Free up space on your device' section (and others) in:

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