You’ll start to recognize the larger instances after a bit. The domain/instance name sometimes has a hint as to what kind of platform it is.

Lemmy gets along well with “Threadiverse” (Reddit-like) instances: Lemmy, PieFed, and Mbin. Searching for Communities (subreddits) will automatically fetch the community list from the other platforms and you can join/subscribe like you would on Reddit. So, it should be easy for you to access those.

If you’re on PieFed, they take it a little further and allow you to combine similar communities across platforms into a single “Feed”. A few 3rd party apps for Lemmy allow you to do something similar. Just keep in mind that different instances will usually have slightly different rules/mods for similar communities.

Lemmy doesn’t interact too well with Mastodon (Twitter-like) instances. But those users can see our activity and comment on posts at will. You can reply to them just like any other user once they’ve commented on a post that you can see.

Mbin instances support a little bit of peeking into the Mastodon side, and PieFed supports it a bit more. For instance, following users will pull their posts into a catch-all “Microblogs” community that other users on the threadiverse can interact with. Ex:
!microblogs@piefed.social

There are even some “bridge” accounts that have the potential to link up Mastodon and BlueSky users… But as far as I’ve seen on the threadiverse side, only PieFed users have recently been able to take advantage of that.

You likely won’t see too many users from Threads. A decent number of instances block Meta and any Threads user would also need to enable something in their account settings to see outside of their bubble.