For example, I’m on Lemmy.sdf.org and I joined the Apple@lemmy.ml community. But there are many missing articles and comments. If I browse directly on Lemmy.ml for the same article there ~90% of comments are missing.
https://lemmy.ml/post/1152794 has 26 comments https://lemmy.sdf.org/post/11232 has 0 comments
You can interact/reply to people from anywhere in the Fediverse.
For example - this is my Mastodon account. I can see your post and reply to it just fine, from Mastodon.