I think if the local and remote instances are federated - for posts submitted to remote communities that have subscribers from the local instance - posts to the local instance can be annotated with cross-posted to: links, whenever the local instance is aware of other federated posts that have a matching URL in other OP posts.
A single OP can manually cross post to other communities using the cross-post button next to the title of a post, although that will auto populate the body text of the new post with quoted text from the original, as well as an embedded hyperlink to the original.
So coss-posts can be both auto detected by Lemmy, or manually created by OP(s).