system
A chat on subreddit /r/pcmasterrace
human
So for the windows 10/11 you can Google and download the free version, which has a few missing features and a water mark but very usable 
You can always upgrade at anytime by buying a license key 
There's legitimate websites that sell them for under 20 dollars
gpt
The websites themselves might be legitimate but those keys, sold in that way, are not legitimate. Not that Microsoft would care though, they’re making money by selling your data anyway.