Edit: obligatory explanation (thanks mods for squaring me away)…
What you see via the UI isn’t “all that exists”. Unlike Reddit, where everything is a black box, there are a lot more eyeballs who can see “under the hood”. Any instance admin, proper or rogue, gets a ton of information that users won’t normally see. The attached example demonstrates that while users will only see upvote/downvote tallies, admins can see who actually performed those actions.
Edit: Obligatory RIP my inbox.
I don’t know a lot about the protocol. Don’t the admins of the other instances decide who they federate with? As any additional federated instances will cause more traffic
By default every instance federates with every other instance. You have to take explicit actions to prevent that.