Instances are more like servers or domains. Your instance is lemmy.world. We can see votes from individual users because in order for votes to federate across instances, (like if you, on lemmy.world, were to vote on my instance, lemmy.dbzer0) they must be public. So anyone with the know-how can inspect who has voted on posts, and how they have voted.

Someone spun up three entire instances and mass created dozens of accounts, just to mass downvote. It’s just blatant vote manipulation, and is why the admin stepped in to defederate (stop sending info back and forth) from the rogue instances.