I believe every post on here goes to Mastodon, as long as the community is being followed by someone on Mastodon or your account is being followed by someone on Mastodon.

From my understanding, you can’t post to an instance, until someone from that instance either subscribes to your lemmy account or the community or search for it. This will force the intance to pull your post to that instance.

I knew mastodon connected to pixelfed, but never Lemmy. Huh.