Earlier quoted context omitted.
AFAIK, you don't. An iPhone will work fine without signing in to any Apple service and you can opt out of all Apple data collection right in the device. I've set up several testing iDevices for work with no Apple accounts. The only thing you can't do without an Apple account is to use the Apple services like the App Store and iCloud and other paid services. I think even software updates can be done without an account…
What good is an iPhone without access to the App Store?
Only when I download I paid app do I want to identify myself so that I can download it again.