Lemmy specific federation, no question is stupid or dumb - how does it work? Each community has a home server...

... which server sends when two peer instance end-users reply in comments to each other and neither is on the home server?

lemmy.ml is home to this community !lemmyfederation@lemmy.ml - if a user on midwest.social comments, which server does the distribution to all the servers subscribed to the community?

If you can point to Rust code in the projects where this happens, please do!