Think of Lemmy as being just a different Mastodon client that happens to display things to their users with a different skin.
You interact with content on a Lemmy instance the same way as you interact with content on a different Mastodon instance.
(Technically they're both ActivityPub clients, for the more correct terminology)
There do seem to be some kinks to iron out between the clients, though. The ! thing might be one where they disagree on how to handle it.