Both Lemmy and Mastodon are fediverse software. So while the uses are a little different, you can use both to interface with the same content.

Mastodon doesn't have a concept of communities, it creates users for communities instead.
On Mastodon the custom is to follow hashtags and users.

What happens when you write a post here in the scope of Mastodon:
All Mastodon servers that are federated with lemmy.world (these can defederate instances same as Beehaw defederated Lemmy.world) will mark that your user (@[email protected]) made a post and the user created for the community will boost it.
An answer made to a post on a Mastodon instance (that is not defederated from Lemmy.world) will eventually be synced to originating Lemmy instance.

Since we don't have a custom of using hashtags on Lemmy, some content is probably getting unseen in Mastodon. But if we started using these, it should start appearing.

But in the end both are separate things, like reddit and twitter are. Separate logins, separate code, etc. It's only the ActivityPub that makes the content cross.

Random example