More storage needed on iPhone 6s

It is not possible to add more storage to an iPhone. All you can do is delete things on your phone to free up storage. Or, buy a new phone with more capacity.

See here:

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

Manage your photo and video storage - Apple Support

iCloud storage isn’t the same as the iPhone internal storage.

iCloud is a digital/cloud-based type of storage meanwhile the iPhone storage is already implemented into the logic board of the iPhone.