You could crawl the Fediverse looking for instances and communities of the sort that your instance understands, but new servers and communities can show up whenever, so you'd have to keep looking continuously. Stuff also gets interesting because different servers have different views of content.

I've seen posts from users on two different servers talking in a thread on a 3rd server asking other users to help proxy messages manually between them because one of the servers was defederated from another and the messages were only going one way between the two users... I'm not sure in that case what the communication pattern was between the three servers (never mind me on kbin.social -- which isn't a Lemmy server at all -- also able to see the conversation), but it seemed like a big headache.

I've manually gone to different instances to make sure my posts show up when I make them; kbin is often pretty bad at getting new photo threads to federate out and sometimes needs a bit of coaxing... Looking at my profile on those instances though shows wildly different thread and comment counts sometimes. As of the time of writing this (before posting this comment) I have 30 threads and 85 comments listed in my profile on kbin.social. lemmy.world shows me as having 30 "posts" (threads) and 78 comments. lemmy.mindoki.com (your instance) shows 0 posts and just 10 comments!

There's also users on Mastodon and Misskey which show up for me as part of the regular experience of using kbin but which are a bit more awkward to interact with from Lemmy, I think? If I manually put in a mastodon.social user's account into lemmy.world's user lookup, for example, I can see some of their posts, but I'm not sure if they would ever actually show up anywhere on Lemmy without manually looking for a user?

Nevermind the other parts of the Fediverse like Peertube and AP-enabled Wordpress blogs and whatever else is out there... You can probably get a decent view of most of the Lemmy/kbin-like communities if you have a good list of servers to scrape community lists from and subscribe to everything you find regularly, but I think you'll still have some problems in practice.