lemmy.world runs the Lemmy software. lemmy.ml also runs the Lemmy software. Therefore, the two sites (and a ton of others running the Lemmy software) talk to each other and play nice and which specific one your account is on doesn’t really matter. Other than when a large influx of new users comes in and we start getting weird bugs, of course!

I remember in your other post you said you were using wefwef. So, let’s get specific and say you want to subscribe to a community on lemmy.ml, perhaps AskLemmy.

In wefwef, navigate to the search tab, type in “asklemmy”, and choose “communities with ‘asklemmy’” from the options. You can then click into the community from your search results, hit the three dot menu in top right, and subscribe.

Your next question might be “how do I know which communities exist so I can search for them in the first place?” and the answer to that one is you can either speculatively run a keyword search in wefwef and see what communities come up, or you can use https://lemmyverse.net/communities