https://reddit.com/r/privacy/comments/v624di/apple_tracks_you_even_if_you_dont_have_apple/
We investigate what data iOS on an iPhone shares with Apple and what data Google Android on a Pixel phone shares with Google. We find that even when minimally configured and the handset is idle both iOS and Google Android share data with Apple/Google on average every 4.5 mins. The phone IMEI, hardware serial number, SIM serial number and IMSI, handset phone number etc are shared with Apple and Google. Both iOS and Google Android transmit telemetry, despite the user explicitly opting out of this. When a SIM is inserted both iOS and Google Android send details to Apple/Google. iOS sends the MAC addresses of nearby devices, e.g. other handsets and the home gateway, to Apple together with their GPS location. Users have no opt out from this and currently there are few, if any, realistic options for preventing this data sharing.
It’s for location services based off of WiFi and such. Pretty much nothing you can do. You can opt out on your devices on a certain network, but even then if your network broadcasts a signal, it’s being scanned and uploaded.
The only thing you can do is prevent your device from connecting to (or even just scanning for) these networks and signals so that it cannot triangulate. My phones battery saver mode and ultra battery modes do this. Otherwise I have to turn off location services, WiFi, Bluetooth, etc. And then why is my phone even on?
Treat your phone like your little spy that you take everywhere with you. Assume it’s always listening and always ratting you out. Operate accordingly.