You're already doing it. Even though your account is on lemmy.world you are posting to a community on lemmy.ml and I am answering from my own server on swg-empire.de.

Your server has its own communities but it also receives all the posts, comments and votes from external communities that are on other servers if someone from your server has subscribed to these external communities.

Mastodon is similar but there you follow people instead of communities. You still can follow Lemmy communities as if they were users to get all their posts and comments but it is clunky. You can post from Mastodon to a Lemmy community by mentioning it in a post with an @ in front. The first line of the post will be used as the title.