I got a notice that my cell phone carrier was charging for additional data storage. I never use this. All my data usage is turned off. How/what is causing this?
iPad/Mac uses iPhone personal Hotspot even if you have Hotspot turned off. If your cell provider allows you to have a personal Hotspot and your data usage is being used yet you have the Hotspot turned off and all your devices have cell data turned off there are some providers that will force the Hotspot to be turned on and use it for data.
Lets say your iPad says to use Wi-Fi and it is signed into the same iCloud account as your iPhone. The iPad will force a connection to your iPhone and turn on Hotspot functions and then connect to your iPhone hotspot. There doesn't seem to be a way to turn off this "feature". Except to not have the iPad or other device log into the iCloud account.