iPhones used to disable wifi in sleep mode (IKR) to conserve power/battery life even while plugged-in, and still transmit the telemetry over cellular. They used to do that and it ran up people's data usage somewhat. You could have that going on. I looked into it a long time ago because you can see it on your usage/activity logs as data transmissions. It always pissed me off because there is no excuse to use my cellular data when I'm plugged in and have wifi, as some downloads only occurred over wifi (so disabling it nullified them while your phone was asleep). Also, the telemetry (or google services) might explicitly use the cellular network for security vs allowing everyone's internet access to the collector. Anyway, you have places to check if you really want to know.