Hotspot
Does the Hotspot function on the iPhone 7 cost any money to use?
Question marked as
How to set up a Personal Hotspot on your iPhone or iPad - Apple Support
If you see the Hotspot option on your phone then it is included in your Carrier Plan.
It uses the Cellular Data Plan from your Carrier.
It does not cost you money. However like all other usage if you exceed your Data Plan Limit then yes you will pay more.
You can check what your usage is in...Settings...Cellular...Cellular Data...Current Period...or your Carriers Web Site.
And you should reset the Statistics according to your monthly plan...Settings...Cellular...Scroll down to "Reset Statistics"