I'm sure this question has been asked and answered many times, but I can't seem to find the info.
My naive understanding is that a given community lives on a certain server. For example, this post is at lemmy.ml: https://kbin.social/m/asklemmy@lemmy.ml/t/107757/Is-anybody-else-more-active-here-then-they-were-on
Then, I figured that when I visit that post from kbin.social, I figured it would pull the comments from the lemmy.ml community and that no matter where you view a post, the comments would (eventually) be identical. However, the comments are very different at https://lemmy.world/post/746839 which makes me think that I have a pretty fundamental misunderstanding with how things work.