@JupiterRowland@sh.itjust.works @fediverse@lemmy.world - https://socialhub.activitypub.rocks is a place where people discuss various ActivityPub implementations and potential improvements - primarily from a technological perspective (how the software is written), not as much from a cultural perspective (how users use the software), so it doesn’t directly answer your question, but it might still be helpful in some cases
ActivityPub addressing is really somewhat complex and pretty much any server is going to abstract it with things like at-mentions, sometimes making different assumptions. When I reply to lemmy posts from my microblog.pub server, I sometimes need to make sure to include both the user (@elena@lemmy.world) and community (@fediverse@lemmy.world) if they happen to be on different domains. Otherwise my server’s addressing won’t send it to the right place. And I doubt other lemmy instances will see this post unless I tag them too.