IIRC Mastodon and Lemmy have a give and take relationship because they’re built on the same software so can interact with each other. I can follow Lemmy posts in Lemmy communities that I follow on Mastodon by using the full @fediverse@lemmy.world address. The posts show up and each individual sub comment shows up on my Mastodon feed and if I want more context I can open up the thread.

kbin.social can receive Lemmy and Mastodon content and reply but Lemmy and Mastodon cannot view posts from kbin.social because the software is just slightly different. If anybody knows it better than me, please feel free to correct me.