How do I buy iPhone storage?

See this Apple articles below

If you need more space for an update on your iPhone, iPad, or iPod touch

https://support.apple.com/en-us/HT203097

How to check the storage on your iPhone, iPad and iPod touch

https://support.apple.com/en-gb/HT201656

Clear Other storage on your iPhone or iPad

https://support.apple.com/en-us/HT211686

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

https://support.apple.com/en-us/HT206504

You can not increase the storage capacity of any iPhone.

iCloud storage is online storage for backup.