The magic of the fediverse is that Lemmy posts are already available on Mastodon!
For example you can see every posts and comments made to this community by opening the @[email protected] profile on Mastodon. It works with Lemmy users as well (@[email protected]).
From there, you can follow communities and users as well as reply or boost any post or comment!
It also works the other way around: you can post to Lemmy from Mastodon by mentioning the community (ie. @[email protected]). Just keep in mind that the first line will be the title, while the following lines will be the post’s body, and that hashtags are rendered as text on Lemmy.
Edit: my links to mastodon.online were redirecting to the Lemmy instances instead of showing the profiles in Mastodon.