Short question is, how do I get my windows 10 product key so I can do a fresh install from a new HDD?

Long question. After upgrading from windows 7 Ultimate to Windows 10 pro. I'm told all I need to do is to create a media ISO & burn to DVD by downloading the ISO using the media creation tool. Ok, sounds fine but what if & this is a likely if my laptop finally dies & I decide I want to buy a cheap second hand one that comes with no OS or one thats not genuine. Normally I would just install my Win 7 on it & activate perfectly fine. But how do I do this with windows 10? I was searching the web & I saw someone say you have to re install your original windows, upgrade & then make a media ISO using the tool & burn to DVD. Then boot from CD to do a fresh install. Surely if I've upgraded from windows 7 to windows 10 legally, I no longer need to bother with my windows 7 or its product key? & if so, how on earth can it be linked to my windows 7 upgrade if I have no product key to enter?

If someone could explain this to me, that would be great!