I think it's local and depends on the server. I can't see Lemmy downvotes from Kbin, but I can see Kbin's downvotes on Lemmy posts. I'm still trying to wrap my head around it myself.
I think I remember reading some instances just turn off downvoting entirely.
I have Jerboa on /e/ OS and I can't downvote. But I'm on the reddthat instance (not sure why) so maybe that plays a role too?
OK its more complicated. I can downvote when I view replies in my inbox. But I cant downvote when I view replies in my profile of when browsing posts in channels.
You can, but I don't think downvotes get federated to other instances. So even if you downvote on Jerboa, I can't see it on Kbin. Something about how the Activitypub protocol works.