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if you can't understand instances, it's just reddit, but volunteer run.
But how come some instances show posts from more communities than others? How come i can register the same username multiple times on different instances? How come it's so difficult to subscribe to a community from a different instance?
you can use the same name because, similar to an email address, the instance is part of your username (eg: [email protected] is different to [email protected]) your app / site may just hide the end part
the subscribing problem might also just be on your app specifically, as i have no problems subscribing
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?)
Beehaw defederated lemmy.world