Lemmy does not, as of now, interoperate very well with Fediverse services such as Mastodon and Pixelfed. Sure, you can follow Lemmy communities from Mastodon, but it's not a pleasant experience. The group just boosts everything that is ever posted to it.
Likewise, Lemmy does not work with Phanphy - it has its own API, and separate apps. It's too different from Pixelfed/Mastodon for it to make sense to share an API.
If you search for @elena at mastodon.social you will, however, be able to see your user from there; you can view this post, and if you have an account you can comment on it and contribute to the discussion like anyone else. You can also boost the post or comments to it, making it possible for content from Lemmy to reach far and wide. We sometimes do get comments from Mastodon users, so it clear that this happens every now and then, but mostly it's kept separate.
Mastodon users can also post to Lemmy by tagging a community (like they would tag an a.gup.pe group), but it's not very intuitive.
We commonly refer to Lemmy as part of the Threadiverse - a subset of the Fediverse which revolves around threaded discussions around shared content (Reddit like). The main platforms are Lemmy, Mbin (which is what I'm currently posting from), and PieFed.
Mbin and PieFed go further in the direction of interoperability than Lemmy does. Mbin supports Mastodon-like microblogging; if you check out the search for the hashtag Lemmy, you'll see not only this post, but also microblogs from Mastodon and all kinds of content. Limited, of course, by what is federated with that instance (Kbin.earth doesn't have too many users).
In Piefed, users can follow Mastodon groups made with a.gup.pe, such as the knitting group. Often Mastodon users start their posts by tagging each other, so it doesn't look completely native, but it can be neat. You can also follow PeerTube channels directly in Piefed.
In short, it's quite complicated - there are different platforms, and they all solve interoperability differently and prioritize it to different degrees. There's always the possibility that Mastodon users will stop by and say hello, but how easy it is made for them to do so varies quite a lot.