Good questions. Its been a bit since ive looked at lemmys code so im going to dive in again.

Here is the docs for the votes themselves. https://github.com/LemmyNet/lemmy-docs/blob/main/src/users/03-votes-and-ranking.md

You can see this in action here: https://lemvotes.org/ if I recall.

I know the vote model is here . You can see the logic here. It checks to see if the post is actually available in the community with other functions and other such checks. It doesnt ask any lemmy instance for totals (unless theres something im not aware of), it tallies based on real time data. Which can also include fediverse actions(not necessarily lemmy/piefed/mbin/etc... but also mastodon, gotosocial, and other such services).

One of the things piefed does a bit differently is it bunches up votes for sending to other instances. And there is a backfill operation when puling from new communities or users. I would argue its a bit better than lemmys system at least from a technical perspective. But its a VERY minor one. Its nice we have multiple ways of getting to the same result-ish.

Hope that helps! I dont know everything but I know a little bit.