The product key comes from the computer's BIOS or input from the user.

The reason you can use the same OEM disk for Windows 10 to install multiple systems is because the disk itself contains no product key or license. This is completely different than previous versions of Windows, which used the product key itself to activate Windows. But you don't use the credit card to get into the event (activate Windows) - you use the ticket (digital license) that you purchased. The credit card (product key) is used to purchase the season ticket (digital license). It's like purchasing season tickets for an event with a credit card. Once the digital license is established by the product key, it is the digital license that activates Windows. A retail product key can be used to establish a digital license for multiple computers. An OEM product key, whether stored in the computer's firmware or purchased separately is only supposed to be able to establish the digital license for the first computer it is used on (although, it is not strictly enforced by Microsoft). The product key only establishes the digital license for Windows 10 that is stored at Microsoft. The product key for Windows 10 is not the license. It is very different than previous versions of Windows. Bugger.Well, to be quite honest and frank, it is because you don't know how Windows 10 licensing works. How can that be? On a separate note, now that there is a 'Windows 10 Home' acknowledgement by my current system, the 'digital license' has not changed. Anyways, as a matter of fact I've built approximately four separate systems and all while using the 'exact' same OEM disc of W10 Pro.

Windows 10 home digital license