I recently did a clean install of Windows 10, overwriting Ubuntu. How did Windows find out that I had activated Windows 10 before on my computer? It didn’t ask me for a product key, but it is activated.
Solution:
Windows DE (Digital Entitlement) takes IDs of different parts of the hardware (I don’t think anyone but MS knows which parts it uses) and creates a hash of it. It then submits that ID to MS activation servers that check the hash in their database. If the ID is found, MS reports that back to Windows, which marks itself as activated