But how.

It hasn’t made a difference for me, I’m on Verizon if that matters. I’ve got Wireguard set to always on and all traffic but I still get the usage notifications. IIRC there’s a separate apn for any traffic that goes through the hotspot or tethering connection and that’s how it’s monitored. The traffic will be encrypted, but they can still see it.

Doesn’t matter, it still gets flagged as hotspot traffic.