The votes themselves are the federated action.

If you fetch an old post, your instance will not see the previous voters. After that, whenever a user votes the instance will get the message "User X@instance upvoted/downvoted post Y" and the vote will be added to the database with the voter's user ID and counted.

This has a practical function. If you don't keep a list specifying who voted for what, it would be much easier to fake votes from one instance to another by simply communicating the message "Downvote post Y". With the current method it is still possible to create a lot of fake accounts and mass-vote, but at least you can get a better insight when looking at the database if the votes are associated with accounts with no activity from a single instance.

There are some federated platforms that will show who likes / dislikes something. I know that friendica used to do this - I have not checked if it still does. So it is not only admins who can see this, this is is basically open information in the fediverse.