You should look in a mirror, because you're the densest one here, you're just not getting it

It's possible with high precision sensors in a device meant for that purpose i.e. NOT your average phone

Its NOT possible with any sort of degree of accuracy with the commodity grade sensors available in most consumer available devices

The "app" that they "pinky promise" is accurate, isn't. It can tell when you're moving and coupled with GPS even what speed and that's about it. Any promises about being able to determine where you are in the vehicle are a lie.

The sensors built-in to your phone are done so for the job of determining the position of the device relative to YOU the PERSON NOT relative to where YOU AND your phone ARE in the physical space i.e. is it in your pocket, are you trying to rotate to landscape, is it upside down. NOT where you and your phone together are in physical space i.e. what seat you're in.