If an iPhone 7 Plus is used as a hotspot for an iPhone 6 Plus, then I think the iPhone 6 Plus will treat the data as "not cellular" and use as much as it wants?
So, for example, Google Photos or other apps, when set to not use cellular data, will transmit data as much as they want when using a hotspot, which happen to be cellular data on the other phone.
That means, Google Photos or other apps can be transmitting 30GB or 60GB of data, and run up the phone bill to several hundred dollars.
Is there a way to prevent or lessen the issue?
P.S. after finding more things out, it looks like one big thing you can do is, say, you set up a hotspot using your iPhone 7, and now use a notebook computer and an iPhone 6S to use that hotspot, make sure you turn off all the "iTunes store automatic app update" on the iPhone 6S, and turn off "Download all updates but let me decide when to install them" on the notebook computer, or else they could download a GB or two without telling you. Also make sure Google Photos is not running in the background, or else it could think it is Wifi and upload the 500MB of photos you took recently.