Mastodon, kbin, lemmy and all the other fediverse apps all use the same api, activitypub. This means we can all interact with each other even with very different ui and content goals. Mastodon doesn't interact with lemmy much right now because the uis don't really mesh very well, but it's possible. If you see a post that has @<community name> in it that's a good sign it probably came from mastodon or similar.