Is W10 free to install on any computer that came with windows?

So recently, I installed new solid state drives in multiple machines. I noticed that Microsoft never asked me for a product key like with older versions of windows. I'm assuming that Microsoft must be verifying these computers somehow. What are the conditions in which you are allowed to upgrade for free?

I have a PC I built that is currently running W7. I like W7 and would like to keep that computer as is. Would I be able to dual boot W7 and W10 without have to purchase a second version of windows?

Hi,

Just hypothetically, if you have a pc with a license for say Win 7 then installing Win 10 on that pc takes that Win7 license.
This turns the old Win7 license legally unusable for deployment on that or any other machine.

That doesn't mean you can't revert to Win7 but you just can't use both simultaneously.

Maybe someone else will correct me, I'm by no means an expert in this kind of matter. This is just how I understand it.

Cheers,