You can follow Lemmy communities in Mastodon by putting the "https://lemmy.world/c/fediverse" in the search box, hitting enter, the community comes up as a group in the search results, then you hit the follow button, and all posts will be sent to your home feed, it's not the greatest interface between the two, but you can post, and reply, like, follow, all that from and to a Lemmy community/user/post
No idea about Kbin, or Pixelfed as I haven't checked them out yet
It'd be slicker than greased pigshit if we could use our single ID like a passport across the entire Fediverse to be able to log into Fediverse federated assets seamlessly, but alas, not yet, small steps
I don't know all the permutations, but Mastodon profiles can indeed be seen on Lemmy. They get parsed as a community where each Mastodon publication (toots?) are a post.
Though comments on said posts will only be seen by people using the same Lemmy instance as you, they do not get interpreted by Mastodon.