I think it's calling a server generating validation data (probably with a pre-set hardware informations to be able to run it on a Linux machine which is cheaper, with emulation as pypush does it or by directly loading the macOS executable in the memory and run the right code snippets there).
I'm curious what are the implications of having pre-set hardware info. Maybe rate-limiting? or easier for Apple to flag those particular serial numbers to block the service if they wish?
