What is the OEM Product key stored in?

On PCs made my Dell, HP, Lenovo etc. You often get a OEM Key, Which allows you to install windows on that particular PC, If you attempt to install windows with that key onto a different PC, it'll not activate. So What is this key stored/registered on? The Motherboard? CPU? Or how does microsoft know that that key belongs to that PC? And if so, If I replace those parts will the key not work?

Thanks in advance!

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