I have only observed Lemmy->Mastodon and Mastodon->Lemmy
If I follow a community, in mastodon it will show as boosts and will show Title + link to the original post with all the comments also as boosts. It has some probability of working. Not always. I couldn't get hashtags to work.
If someone mentions some lemmy community it will get federated into that community but only into first mentioned community. The first line of a microblog will be set as a title. When users write hashtags and mentions first, it will be unreadable. Not all replies will get federated back and forth.
P.S. I have seen kbin [R.I.P] (Long Live Mbin) posts on lemmy and it worked good
~~You can federate and respond but not really follow.~~