If you use the stock ROM then they probably can. A fey years ago the carriers would put proprietary hotspot apps on your phone which would integrate with their billing system, but if you switched to another ROM you could just use the built-in Android hotspot option and it would work just fine.
With an AOSP ROM like LineageOS they can't detect it from the phone itself, but there are still some differences in the packets that a phone would send vs the packets a computer would send (you can Google about how to hide tethering on T-Mobile and find some discussion on that); it's not very hard to change your settings to avoid that method of detection.
With an AOSP ROM like LineageOS they can't detect it from the phone itself, but there are still some differences in the packets that a phone would send vs the packets a computer would send (you can Google about how to hide tethering on T-Mobile and find some discussion on that); it's not very hard to change your settings to avoid that method of detection.