Most of the time, tethering is allowed in the base package. What happens afterwards depends. They can either cut you off or throttle. The only experience I have with this is Orange, and they throttle to the point where you basically have no connection (SSH is unusable).

Not sure though how they differentiate the traffic. There are "access points" set up which I presume are used for either connection. In my case they are the same but I seem to remember on an older phone they used to be different. It was also a different provider. I'm also not sure how they would detect that they're changed.

Screenshot of my setup: https://imgur.com/a/xH88Itp