That's really surprising!
But it's certainly not an official Microsoft key.
I'd like to know what it is and where to find it?

Not really surprising: the thing is known and quite normal in fact. I bought mine from the Manor site for 9.95. If you type in Windows 11 Pro, you'll see that there are lots of prices like that. Manor is the cheapest I've found. To install, the best thing to do is go directly to your Windows update and give the key you received. When you receive the download e-mail, you'll even be given a link to the procedure to follow: it's that simple! As you can see, I'm taking the time to reply to you, despite the dryness of your message. I hope you have a lovely day. Words like hello, thank you, please are so beautiful.