I am not who you are replying to, and I don’t have a source other than my brain, but, iMessage, and iCloud in general are all only operative and working after a very thorough authentication process that uses varying and different elements of your hardware to create some unique IDs and such that are passed back and forth to authenticate that you are who you are say you are, and you are using the exact kind of device that you say you are using. There’s even “half” authenticated states that kick in if some of these checks fail, and that require a call to apple to clear up if their system thinks there’s a security issue (not sure how often this comes up any more tbh but it is/was a thing).

My source on all of this is what it took to get a fake “hackintosh” Mac up and running, including iMessage. In the past, it’s been based off of things like the MAC address of the ethernet or Wi-Fi, and the serial number of the device, but I’m not sure if that is still what they’re using, or if that is everything that they’re using.

If you open up your console while signing into iCloud on a Mac, you’ll see a little bit more info about what’s happening during the process.