NEW PC: can a bought Win 7 licence be used to upgrade to 10 or...

I am building a new PC for personal home use. I've been told I could use my laptop license for it. And elsewhere I read I can't do that. I've also read I could buy a windows 10 license key on Amazon and then use that for installation and then upgrade. But here too I found that there may be problems doing so as well. Hence, my seeking advice here.
Anyhow, I don't know who to believe or what to do. Any advice, the more the merrier, would be appreciated.

(BTW even if the retail price of Windows 10 were half its price I would have found it too expensive so imagine what I think of its current actual price. And now I am wondering if Linux is my actual only option)
The laptop was originally with a win 8.1 version which was updated free to W10 when it first came out)

Windows 10 home ver 1903 (build 18362.720)

Fun note: Back in the days when PCs were 1st introduced to the general public I was shocked to hear that the computer that I was buying would not work without an OS. I'm thinking how can you buy something if it doesn't work on its own; and why do I have to buy something else to make it work? My reasoning was, "It's like buying a radio but you can't hear any radio stations unless you buy something that makes it translate the received signals into audible audio." The PC was after all the first type of electric device where it did not function on its own. And I was surprised that nobody questioned it and just accepted it as a norm.

I don't know the situation in Canada as regards moving software.

But if you want to buy win10 there are deals around.

Creativemark Store

If you are going to keep the laptop and use it in addition to the new computer, then yes, you must purchase a second Windows for the new computer.

Hi thanks F14tomcat I await your friends' replies.
SIW2 your link takes me to the checkout page and the store logo as a link does not work. Also if your suggested store is good then what of Amazon, as I had mentioned.
NavyLCDR thnx to you too.

Computeractive Software Store - Windows 10 Home - 79% off RRP

clicking the buy button takes you to cleverbridge who handle the sale where you are offered pro edition for an extra five bucks
You need to select your currency in the dropdown at the top of the page. That is the cheapest I have seen where you know it is legit. Are amazon doing such a good deal?

SIW2 said:

Computeractive Software Store - Windows 10 Home - 79% off RRP

That is the cheapest I have seen where you know it is legit. Are amazon doing such a good deal?

How do you know it is legit? The webpage says you get "You are supplied with a Windows 10 Home activation code and the official 'Windows 10 Media Creation Tool'." So....where did the "activation code" come from?

We know it is legit coz it is Denis publishing.

Brands - Dennis Publishing