I did some digging around in an old jailbroken iPhone (iOS 12) to try to figure out how to stop broadcast alerts from the cell towers and there is a database on the device itself with various parameters from cell carriers around the world. So there are least some settings that are “hard-coded” in each OS release which don’t have to be sent from the carrier - the iPhone will do X based on the SIM in the phone or the network it’s roaming on.
Not sure if tethering is one of the parameters , but there were quite a few.
Yes tethering is absolutely one of the parameters - including whether it’s enabled at all and whether it goes over its own separate APN (which is how the carrier can limit it or throttle it separately from the main data plan) or is sent in-band over the main APN used by the system (alongside other params like the number of client devices connected to the hotspot if I remember correctly). Android has a similar “carrier settings” database, although I haven’t looked too deep into what kind of settings are available.
Not sure if tethering is one of the parameters , but there were quite a few.