How do I transfer Office 2021 LTSC Professional Plus from laptop to a new desktop computer when uncertain which of multiple product keys has been used?

I appreciate the time you took to answer, but this answer is not only not useful, it leaves out important information that I myself found via web search, and then recommends a path of action that is irresponsible or even negligent.

Apparently recent versions of the Office standalone product only put the last 5 digits of the product key in the registry. This is fine; I have two keys and I'm just tyring to figure out which one actually applies to the old computer. Because of this change, however, most office key-finders no longer work, so the suggestion is not particularly useful on the face of it.

The worst part, however, is suggesting to a user of unknown technical competence (fortunately I am competent) to just 'google for a piece of software, download and run it" with absolutely no reference to any reputable tools for this purpose. The internet is full of tools that masquerade as useful, and perhaps even do what they say, but also do more malicious things they do not reveal to the user, from capturing the decoded key to use elsewhere to installing malicious software. This suggestion is thus, at best, irresponsible, and at worst, negligent. While I appreciate the time spent to provide a reply, this is not useful and is, in fact, hazardous.

Now if you have a link to a reputable piece of software that meets your description, please I would like that very much. I am not, however, going to just download random software to work through the registry of my computer because it was one of the top results on Google. A simple PowerShell script that can do this work would also suffice. Otherwise, I have to mark this answer as unacceptable.