Bro, I'm on Lemmy, Mastodon, Bluesky, and Threads and I'm not sure I even know what exactly it is and how they talk to each other. I just use the service.
tbh I never quite understood the email analogy because emails are a completely different format, even if the general idea of communication across providers is the same. Comparing it to interoperability between messengers (e.g. accessing Signal groups via Telegram, if that were possible) made it way easier to understand.