My IPHONE 6 is maxed out at the 16GB and that is all that is available. I am paying Apple monthly for additional storage. What am I paying for?

There is no way to add more storage to any iOS (iPhone/iPad/iPod) device. When you purchase more iCloud storage, you are paying for more space to hold backup data in iCloud. That has absolutely nothing to do with the physical storage on your phone.

This is your second post on this very subject today --> https://discussions.apple.com/thread/251438084

Were you hoping you'd get a different answer that better meets your needs? The answer will be the same no matter how many ways and times you ask this question.

iCloud storage which does not add to the storage on the device.