Mostly answered in my other comment but for visibility and completeness, I’ll try to answer it here again.

The content may look different on the surface but that’s mostly because the different applications present them differently. Under the hood, it’s almost identical. Users create posts, other users can reply or like. What differs is the details. Mastodon posts are by default limited to 500 characters, Lemmy/kbin posts can be a few thousand, Plume posts can be even longer. Mastodon and Plume only have likes, Lemmy and kbin add dislikes (downvotes). Posts and comments that come from a different application may look a bit weird in whatever you use but at least they show up in your feed and you have the option to click a link to see them in their original form. In facht, @eatyourglory@mastodon.uno’s reply to your comment came from a Mastodon instance and they will see my reply in their Mastodon feed.

As for the difference between kbin and Lemmy, they’re two different pieces of software that interact with the Fediverse in a very similar way. When someone wants to setup a Reddit-like fediverse instance, they can freely choose between them based on personal preference (I chose Lemmy for my instance because kbin is harder to install and update). Imagine them like being able to choose between a phone by Apple, Samsung, Huawei or Nokia. All those phones have their own specific pros and cons but because they communicate through the same protocols, they can still talk to each other. An instance would be analogous to your Samsung phone or my Apple phone or my partner’s Nokia phone. There are many fediverse instances that use Lemmy as their software same as there are many Samsung phones in the world. The fact that most of them have very unimaginative names (looking at you, https://lemmy.ml/ and https://lemmy.world/) doesn’t help but as positive examples like https://beehaw.org and https://feddit.de. Both use Lemmy and can talk to any other Lemmy instance but their names make it clear that they are their own thing.