You can probably just enable Airplane Mode on your phone and expect that to work fine.
If there is a secret involved in this it would be undocumented commands sent by the network to phones that then interpret them as commands to connect to the network, plus baseband processors the periodically look for such commands. They don't radiate, so very little power is used, but they should be entirely dormant.
If the baseband is still powered, I can only assume it is simply sending (or responding to) periodic commands that do not require network authorization, which could possibly still let the network know where you are even without involving any roaming charges.
https://9to5mac.com/2021/07/21/ios-15-here-are-the-devices-t...
A light sensor is used determine the amount of sunlight streaming and heating the interior.
GPS data is used to help calculate the angle of the sun, and determine which areas are likely to be heated the most, and thus needs more cooling.
A compass can be used to make it work even when the car is stationary (when GPS heading is unavailable).
A singular interior temperature sensor can be used for feedback, injecting some truth to balance things out.
It may seem far fetched or even fanciful -- functionality that only exists on very expensive or exclusive brands, or very new vehicles.
But as long as the GPS receiver and nav system exist on a network, and multi-zone HVAC is a thing in a given chassis, then the rest is mostly just software that is free (for the manufacturer) to copy and use.
Thus, Honda has been doing this stuff for at least twenty years now on fairly pedestrian vehicles.
The Honda factory service manual for my 2007 Odyssey touches on these operations, including diagnosing issues. It does not have an ISBN number that I can find.
These systems (multi-zone HVAC, navigation) seem to have been introduced in MY 2004, remaining mostly unchanged until MY 2008 when (at least) the navigation was facelifted and the rear HVAC zone became more autonomous.
Anyway, within volume 2 of that manual is this passage:
i-Dual Zone and i-Tri Zone Climate Control system (With navigation system) The "i-Dual zone" climate control system automatically controls the temperature and the vent mode of the air direction to the driver's and passenger's side, depending on the angle of the sun, and the direction of the vehicle. It calculates information such as date, time, longitude, and latitude from the navigation system, as well as the radiant strength of the sun from the sunlight sensor, to determine the appropriate mode position and temperature to be directed to each side. The "i-Tri zone" climate control system is an addition of the automatic controls with the temperature and the vent mode of the air direction to the rear section to the "i-Dual zone" climate control system.
...and if I'd read something like that in 2004 (or in 2007), I simply wouldn't have believed it. Even now in 2024 it seems either a bit far-fetched or blatantly obvious, with very little middle-ground -- especially for a vehicle that can't even turn its own headlights on and off or adjust dash lighting intensity based on apparent daylight conditions (as Chevy and BMW were variously doing over a decade prior).
(Other parts of the Honda manual describe the in-car temperature sensor.)
Regardless, most of my around-town trips I do by bicycle.