How does interoperability work between different fediverse services?

Possibly! I have replied to this post from my Mastodon account (hopefully that post will show up soon). People on Mastodon can see these posts and interact with them, but it's kinda clunky. The posts and comments are implemented as boosts. So you post here and I see this on Mastodon:

Every reply will also show as a boost from nostupidquestions@lemmy.world.

I don't think it works the other way around. I don't believe you can follow specific AP accounts from here (like you can't follow my Mastodon account from your Lemmy account). I think they have to be groups/communities. Someone please correct me if I'm wrong.

You are. Lemmy is kinda the odd one out but mastodon/peertube/misskey/etc... all use activityhub to communicate.

Parts of the protocol for Lemmy kinda stopped working for Peertube stopped working 7ish months ago. And Lemmy treats posts/upvotes/down voted differently than the rest of the fediverse. Just some small changes to be aware of.

Commenting in part to save this thread for later reading when someone more knowledgeable gives a full answer.

Here is what I (think I) know:

• It is possible to follow Lemmy users on Mastodon, which gives you threads they start in your feed, but not comments they make (I have not really tested this, but this is what I remember reading).
• It is possible to follow Lemmy communities on Mastodon, which makes those communities appear as bots that share everything (posts, comments) posted to them (I have done this accidentally before, not realizing that an account I was following was actually a Lemmy community, then wondering why it was only boosting Lemmy posts until I figured it out).
• It is possible to post to Lemmy communities from Mastodon. I don't know how exactly, but !kde@lemmy.kde.social is a community that gets a lot of posts like that, e.g. https://discuss.tchncs.de/post/27370850?scrollToComments=true - looks like they simply tagged the community? I am not sure how this works and have never done this.
• If you are seeing a Lemmy thread or comment on Mastodon through one of the above methods, you can reply to it; you can usually tell when someone is doing that when they put @ links to the people above them in the thread into their posts. Here's a thread full of examples: https://discuss.tchncs.de/post/27508474?scrollToComments=true

Thank you for asking this. I've been wondering the same question for quite a while but have been too lazy to actually find the answer.

Masto to Lemmy works OK. Lemmy to Masto not at all. I have a hazy memory of someone posting to Lemmy from a Friendica instance but might be wrong.

Generally, its a bit indifferent.