The free Windows 10 license you receive is tied to your PCs hardware. Youre still allowed to use Windows 10 on that same PC even after changing its hardware, but that may not be clear. Microsoft wont give you a product key.
Microsoft really doesnt want to explain how the activation process works. It doesnt want to give pirates too much information, but that does make the licensing process confusing for legitimate users, too.
Should answer the questions of a few users here.