How can I buy more storage space

You can buy more iCloud storage space but not more space on the iPhone itself. See: What's the difference between device storage and iCloud storage? - Apple Support

To buy more iCloud space, see instructions in: Upgrade your iCloud storage plan - Apple Support

While you can purchase more iCloud storage, that does not increase the storage of your iPhone. That is syncing and backup storage in iCloud. See this, What's the difference between device storage and iCloud storage? - Apple Support

To purchase increased iCloud storage, see this, Upgrade your iCloud storage plan - Apple Support