It does something that allows it to communicate with Microsoft's servers before the actual install commences. There is no other way that one could reinstall on a wiped drive without a license key, and there are multiple accounts of people doing just that on these very forums.
If one is booting from the install media, which you are when dealing with a drive that's a "blank slate," there has got to be a way that Windows 10 checks to see whether the motherboard on the computer in question has had a Windows 10 license activated for it in the past. This would be true even if Linux were on the drive at that moment in time (or Windows 10, for that matter, since it's not using anything on the HDD/SSD itself when installing from bootable media).
My main point, though, is that one does not choose Home or Pro when downloading the Windows 10 ISO. I believe that one can choose 32-bit, 64-bit, or the combination ISO but the last time I downloaded the combination ISO it was too large for burning to a DVD. You can actually upgrade from Home to Pro on a Home machine with no internet connection, because all the components for Pro (though not necessarily their latest versions) are on a Home machine, but they are not activated under a Home license.
Windows 10 is essentially one OS that has component parts that can be activated, or not activated, depending on how it was licensed. This, also, is a change from how prior versions of Windows worked prior to the Windows 10/Windows as a service era.
Another thing it could be doing is installing "unactivated" and doing the activation as soon as a user sets up the internet connectivity after logging in. Windows 10 will install without a license key in 30-day "demo mode" but, of course, if it finds a license key related to the motherboard on the machine it activates itself.
Edited by britechguy, 06 October 2017 - 09:00 AM.
Note about "after the fact" activation.