x-apple-i-md-m is a quiet but critical part of Apple’s model. It allows Apple’s servers to identify and authenticate a device without a user login, cookie, or certificate—just a time-based, device-specific hash.
When an iPhone sends a request to https://guzzoni.apple.com , https://api.smoot.apple.com , or even during iCloud syncing, you will see this header present. x-apple-i-md-m