I've never understood how this can be limited in practice: surely as far as the carrier is concerned, all traffic from the mobile device is the same (unless there are identifiers on the traffic coming from hotspotted devices via the mobile device). Here in Australia we've never had any form of hotspot detection/segmentation - if you have a data plan, all data features work (across all carriers). I do recall lots of online chatter from the US though, especially years back when mobile data was more of a precious resource.
> surely as far as the carrier is concerned, all traffic from the mobile device is the same
Going on a bit of a tangent, but deep packet inspection can identify packets routed using NAT, so if the phone is operating as a typical hotspot it would be identifiable by your carrier. Carriers in the USA used to block / denylist / charge extra for tethering using this exact approach.
I recently switched to a carrier (Fido/Rogers in Canada). My plan limits hotspot by disabling the hotspot settings on ios. However, I was able to enable it again by changing the access point name.
On android, there is an OS-level feature that checks the cell tower to verify if you're allowed to create a hotspot. It runs whenever you try to enable the hotspot feature. On rooted systems, you can disable this check. There are also apps that let you run a hotspot without using the OS feature, bypassing the check.
I believe there’s some stuff like that for commercial things. One project I worked on used an ‘IoT portal’ for cloud based telemetry (at the customer’s request) and we had to get a special SIM card for it (although I don’t know if this is still needed.)
Your phone voluntarily tags the hotspot data with specific TTL values which carriers use to segment the data. Not all carriers work the same though.