Technically, any Lemmy admin can see every user's upvotes and downvotes — even users on different servers, because that information is federated.

I think there are some sites that expose this publicly as well. So you could probably sort it out.

Lemvotes will show you who's voted, and which way, on any post or comment. By "brigading", are you thinking of activity over more than one post or comment?

Mods and admins can see who voted what. For mere mortals it's harder, but still possible because it's all public information.

Your lemmy instance will send all of your votes to other instances to be counted (this is not anonymized), so any foreign instance will have your vote history. Sites like lemvotes can then show that to the world.

Besides Lemvotes, Mbin instances let anyone see upvotes.

As a note, lemvotes doesn't always work. There's times it throws errors. But it works most of the time for filthy casuals.

as has been mentioned lemvotes is the public visible one, but I suspect that most larger admin teams will have access to custom queries from the backend.

I'm learning some deeper SQL for work, so I wrote a back end DB tool as a learning means that can do lots of things like user and instance vote tracking over time: