The "showing more communities" part is called federating. basically, an instance (@lemmy.world in ur case) has a bunch of communities. now the owner of that instance can "federate" with another instance, so that you can additionally view their communities as well.

I'm a complete newb with Lemmy as well, but as far as I know, lemmy.world for example only defederated some extremist instances (and threads if I'm not mistaken?)