How yo buy more storage

See -> What's the difference between device storage and iCloud storage?

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

See also the following Apple articles:

Upgrade to iCloud+ -> Upgrade to iCloud+ - Apple Support

iCloud+ plans and pricing -> iCloud+ plans and pricing - Apple Support

How to use iCloud on iPhone -> How to use iCloud on iPhone – Apple Support (UK)

Manage your iCloud storage -> Manage your iCloud storage - Apple Support

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

Whatever amount it is when you bought it is all it will ever have.

iCloud storage is online backup.

it doesn’t have any effect on the amount of storage on your iPhone.