1. Try to obtain an Apple Security Research Device (SRD)[1] or a jailbroken iPhone.
2. Set up TLS MITM between the device and the Internet. Install the cert on the device.
3. On the device, disable certificate pinning for Apple's apps.
4. Use the device normally, observe all the telemetry flowing to Apple.
You might be a little shocked at the amount of... personal and usage details sent to Apple FQDNs.
Remember: Privacy is a feature of the Apple Marketing dept -- not SWE!
1. https://security.apple.com/research-device/