It doesn’t and the answer is not really. You can post to lemmy communities by tagging them like a user and see the first level of comments but you can’t reply to them or make your own comments. Piefed might have some kind of extra federation integration built in but lemmy does not. A friend of mine wrote a script that will enumerate all the links on a pixelfed instance and then search them on lemmy to manually pull the content but its clunky.