Do you mean at the technical level or UI level?

At the UI level, just have a look at https://kbin.social and you'll see a lot of very familiar posts. Also their posts show up here, just like any normal post.

Mastodon is a bit trickier. If you put hashtags in your post text, I think it shows up like a "toot" for users following it? Sorry, not a Mastodon user, don't really know how it works.

At a technical level, Lemmy is built on the ActivityPub protocol. It's how Lemmy servers talk to eachother, it's how Mastodon servers talk to eachother, and it's how a hell of a lot more services talk. Best analogy I've heard is ActivityPub is like the email protocol for social media platforms.