What is the relationship between mastodon and lemmy.world
I have an account on a mastodon instance and I read that I could access the threadiverse from there, but I didn't find any way to do it that works. What is the relationship between these things?
They are on the same "network", but that only means they use the same format to exchange messages. It does not mean that they work well together. The Lemmy UI is much better suited for posts with large discussion threads.
Basically you can treat them as separate for all intents and purposes.
Mastodon is free open source software that allows anyone to run a social media website similar to Twitter. The site can federate (share content and interactions) with other sites via the ActivityPub protocol.
Lemmy is free open source software that allows anyone to run a content aggregator website similar to Reddit. The site can federate (share content and interactions) with other sites via the ActivityPub protocol.
The ActivityPub protocol is like the protocol that runs email (Simple Mail Transfer Protocol, SMTP). Any servers using SMTP can exchange email with any other server also using SMTP. However, the ActivityPub protocol is pretty complex, and software using it can implement it in different ways. Mastodon, Lemmy, and other software using ActivityPub don't always interact with each other seamlessly.
One day there may be an overarching "fediverse" app that allows users to navigate through all popular software using ActivityPub, but right now you're best to create a different account for each software.
Search @communityname@instance.domain in Mastodon. It should come up with something. That said, the latest Lemmy version has a known bug that temporarily breaks this in some cases.