They all use the same protocol, but developers pick and choose features and how they are presented. They may be able to talk to each other but the experience also may not be ideal, especially if they use different features in different ways. For example, until a few days ago kbin upvotes didn’t show up in Lemmy at all, and Lemmy upvotes did not show as upvotes in kbin, because they used different features to describe them (kbin used boosts, which exist in Mastodon but are not implemented in Lemmy).