Can’t I buy storage ?

hello,

you can not extend the storage capacity on your iPhone since the the storage your iPhone comes with thats it you can though purchase more iCloud storage> https://support.apple.com/en-us/HT201318

also you can optimize your iPhone storage to free up more space > https://support.apple.com/en-us/HT205703

and you can also clear "other" storage on your iPhone> https://support.apple.com/en-us/HT211686

Buy a new phone. Your phone will never have more storage than it did when you bought it. What you can buy is iCloud storage, which is used for backups and syncing. See—>iCloud Deconstructed - Apple Community

You can't buy extra device storage without buying a new device.