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.