Think of them like email providers. If you have a gmail account you can send messages to yahoomail users without having a yahoomail account, even though they are wildly different email services. Same concept for all the lemmy and kbin servers.
@DamnCatOnMyDesk Are you sure that people can actually send messages between apps? I mean, for example, between lemmy and kbin and mastodon?
It seems rather like kbin users can view stuff from other instances and instances of other apps but any comments or messages are saved in your instance as comments on posts of other instances. People on the receiving instance can't see your posts or messages.
Please correct me if I'm wrong. But this is what I've noticed.
This only happens if the instance on which you're posting has defderated from your instace. So, you can see posts and comments on that instance, but people on that instance can't see posts or comments from your instance, includimg your own posts.
It's not just limited to Lemmy and kbin. Technically any Fediverse-compatible site can interact with them (ie. Mastodon, Pixelfed, Calckey, etc). It's just nicest to do it from Lemmy or kbin because they are the most Reddit-like.