I already explained this in an earlier post and I also have posted a tutorial for a clean install in an earlier post as well.
1. Retail off-the-shelf computers that come from the factory with Windows 8.1 or Windows 10 have the product keys for that version of Windows stored in their firmware (BIOS or UEFI). Windows 10 will read this product key during setup (even if it is a Windows 8.1 product key) and if the user does not intervene by modifying certain setup files on the USB flash drive, Windows setup will automatically install the correct edition of Windows 10 to match the product key and it will be activated. This is completely different than the OEM:SLP activation method of Windows 7 that @
Megahertz incorrectly applied to Windows 8.1 and Windows 10. The product keys stored in firmware for Windows 8.1 and Windows 10 allow for clean, vanilla, non-factory activations of the proper edition of Windows whereas the OEM:SLP keys from Windows 7 did not. The Windows 7 OEM:SLP keys only allow factory images to be activated. That is why Windows 7 computers came with a COA label with a product key printed on it and Windows 8.1/10 computers do not have product key labels.
2. Even setting aside fact #1 above - when Windows 10 is first activated on a computer, if the computer is ever connected to the internet, a digital license for that version of Windows 10 for that computer is stored on Microsoft Activation Servers. This is based on the unique Hardware ID of the computer (not just the LAN MAC address as @
Megahertz incorrectly posted). So, even on home built computers which do not have product keys stored in firmware - all subsequent clean installations of the same edition of Windows 10 on that computer will still activate without a product key by retrieving the stored digital license from Microsoft when Windows sends the computer's Hardware ID to the activation servers.
3. The clean install tutorial is:
Clean Install Windows 10
Notice this quote from the tutorial:
"If this is an OEM PC that has a valid product key embedded in its UEFI firmware chip, then you will not be prompted to enter a product key. Windows 10 will automatically use the embedded product key if its valid for the edition being installed. If needed, you can change the product key after installation has finished."
On your retail computer, a clean install from a standard Windows 10 installation flash drive will not even ask you for a product key or which version you want to install.