As far as I understand ActivityPub (which, admittedly, is only very rudimentarily), downvotes are an action like any other: you perform them on your instance, then the instance sends them to the instance the community is hosted on, then that instance sends them to all instances which have users who are subscribed to it.
An instance which has disabled downvotes will ignore all of that, i.e. will not store downvotes or forward them to anyone. If you look at beehaw.org you can see all posts and comments are displayed with zero downvotes, even if you view them from another instance.
"how that functions exactly" - well, you just change the code to ignore it when a user does try to downvote something, don't store it in the database or forward it to other users/servers...
I have previously had the thought that maybe Pokémon and Harry Potter are the same universe.
Pokémon are just Japanese magical creatures and Japanese wizards prefer to train magical creatures instead of doing wand magic like western wizards do.
If you click "Instances" in the page footer and then switch to the tab "Blocked Instances", you'll find which instances your server has blocked, you can't see any posts or comments by users or in communities on instances on that list.
You may be satisfied with that practice or might not, the point is that the fediverse enables you to choose.
My understanding is that many people in the US and Canada identify with their ancestry? Lots of Americans think of themselves as "Irish" for example, some of them may actually be immigrants from Ireland, others may have more-or-less distant Irish ancestry, so this is what may be going on here.
Everything posted on the public web is potential AI training data, federation is completely irrelevant to that.
The rest of your questions has the simple answer that a priori there is nothing "stopping" any of that. You should choose an instance whose admins are looking out for things like that and keeping your experience enjoyable, banning spambots or defederating from spambot farms when they are discovered.
Lemmy is easier to use than the UBB forum I started to become active on at the age of 10 that was the entry point for me being on the Internet at all. Back then we didn't have Markdown, but BBCode, and I had that figured out after a few weeks.
I suspect OP wanted to know about a video platform where they were not getting any automated recommendations that they hadn't chosen to follow... my point is, chronological sorting is still an "algorithm".
There aren't any because all ways of displaying any data at all involve an "algorithm". There are no "algorithm-free alternatives" to anything at all except not using computers at all.
Maybe think about what you're actually trying to ask, then ask that question again.
It's been a long time since I watched it, but this video https://www.youtube.com/watch?v=rStL7niR7gs contained some interesting information that helped me think about the topic better; maybe that will be true for you too.
Not so in Austria. Only Haftpflichtversicherung is required which only covers the car owner's liability.