Basically your post is already "posted" there. Anyone with a mastodon account on an instance that federates can see it. They could either paste on their mastodon search interface the url to this post and see it appear as a mastodon message. They could then reply on mastodon and it would show for you here on your lemmy instance.
They could also follow your account on mastodon or a community just as if you were a mastodon user and then see your posts on their timeline.
If a mastodon user comments on AA Lemmy post, and a Lemmy user replies, it will be delivered to the mastodon user. I've not tried posting to a mastodon user directly though.
If you mention a Lemmy community in a post on Mastodon, it appears as a post in that community. Like this post on Mastodon: it also shows up in the Lemmy community "Linux", and many if the comments are from Lemmy users: https://social.edu.nl/@foxy/111937325118529631
I don't think you can do microblogging like on mbin, but I think you can directly write and reply to Mastodon users, and posts with hashtags should also be able to be found by Mastodon users if they look for them. I don't think Lemmy threads have a hashtag field like mbin either though, so you would have to manually type them out when making a new thread.