Earlier quoted context omitted.
I had the misfortune of charging my iPhone via usb connected to my laptop while working. This triggered the 4G tethering, so i spent an afternoon working on my cellular data plan... schlepping multi-gb csv files around. I didn't even notice anything, until my data plan ran out. I honestly wouldn't mind slower than 4G speeds. With 4G it's already bordering on ridiculous how quickly you can burn up a data plan.
Your laptop chose to override an existing WiFi/LAN connection to use a tethered phone USB connection? That seems highly unusual. I plug my phone into my laptop all of the time, and the tether has never overridden my WiFi. I've never had to use it, but macOS allows you to order the available networks according to your preference. Did you perhaps do this and forgot about it?
Re: iPhone 12 and iPhone 12 Pro
#1001Not on macos when this happened, Linux mint cinnamon... it definitely prioritizes my phone above the wifi