After 20 GB per line, you may experience reduced speeds for the rest of the bill cycle.
There are no additional fees for using your device as a mobile hotspot. After 5 GB of mobile hotspot data use in the bill cycle, mobile hotspot speeds are reduced to a maximum of 600 kbps for the rest of the bill cycle. Mobile hotspot data counts toward your 20 GB high-speed data allowance.
It does use the Verizon system of towers/antennea.
They do not, however, include roaming data in Canada (Like Verizon does on their unlimited plans). A big thing for us, living on the Canadian border.