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 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.
Welcome to ActivityPub! Nearly all of the Fediverse can talk to each other in some way, the main barrier is that a lot of the apps are in early development and you are fairly limited in how those interactions can take place, but that will come in time. You can follow your favourite users and communities in Mastodon and comment from Mastodon, but you are limited vice versa at the moment.
Edit: I found my Lemmy profile in Bookwyrm, but not much there, and it looks like Pixelfed displays the images you post in your feed. :)
Fun facts, WordPress also uses ActivityPub. WP comprises 43% of the internet.
