Lemmy and Mbin are roughly the same thing - "Reddit"

Mastodon is "Twitter"

... and there are more, with Loops being "TikTok" and Pixelfed being "Instagram".

They can all see the same content, but presented in ways that emphasize their unique aspects. That's what people call "Fediverse" but it's much simpler to just think of them as separate apps and ignore that technically true but confusing aspect.