A good chunk of that is phone payments, not the actual service cost.

I own my phone and from the big carriers two phones or more generally costs $80+ a month.

I think those are family plan prices. I pay about $140 unlimited cell data, 15Gigs for tethering, an Apple Watch, and 2 lines of unlimited talk/text.

How would the carrier know if you use your cellphone for tethering, and separately bill that traffic?

User Agent sniffing, traffic analysis, port sniffing and new phones might even tell them. The carriers are already doing a lot of load balancing among all the devices attached to their network. It wouldn’t be too hard to detect.

Years ago AT&T sent me text right when I connected to my hotspot. I can’t see the tethering data use on AT&Ts dashboard but I can see usage per line.

$70+/mo major provider "unlimited" plan + $50+/mo flagship phone

Monopolies mostly. The US is big enough that there are really only 2, maybe 3 providers. Everyone else is a reseller.