I have just resurrected an old laptop gifted to me ... came with no hard drive.
I installed new drive, partitioned and formatted it etc etc, then down-loaded latest version of Win 10 onto USB drive.
Everything works fine, so went to Activate using Product Code from another dead & disposed of machine ... however I found that it was already "activated with digital licence".
I have never obtained any dodgy Product Codes etc for Win 10, and all media used was squeaky clean, so where did this one come from? I gather digital activation takes place when one signs in to Microsoft account during the Win 10 install, which I did.
I've checked the 'new' Product Code against all the others I have used or have on hand, and it does not match any of them!
Mystery? 🙃