I can't describe what "official" mean here, maybe the one that actually came from reddit

Official means nothing here my dude. The communities from reddit have exactly as much claim on legitimacy as one you founded 5 minutes ago.

I just subscribe to everything that looks vaguely interesting, personally.

I don't think "official" means much here, but the one on .ml has 4 times as many subscribers as the next one down, so probably go there.

The whole point of Lemmy is to be federated, yes. That means anyone can set up the same community name on a different instance and be accessible to everyone. If it has more/better content or better moderation, it will survive. Maybe some people object to being on the .ml instance in the first place.

This is why we need federated comms.

Each instance has a low-activity community under instance.tld/c/community. For most instances and most topics, this will mean checking a comm yields maybe a few new posts a week, and if you want to see all the content related to that topic, you'll have to check every instance your server federates with individually.

Having a single instance.tld/f/community that aggregates all the per-instance comms so there's one topic-specific place with everything would be far more enjoyable of an experience.

This functionality is probably the biggest benefit that could be facilitated by federation, since we already have federated post comments. Activity is high enough now that posts feel lively, and the main feed has enough interesting stuff, but the one thing that still feels lacking compared to reddit is being able to browse a sub and find a reasonable amount of fresh content if you haven't checked back in the last week or so.

Subscriptions are also far less useful if you have to manually find and subscribe to every instance's comm with the same name. Being able to sub once to the federated comm and sub to all the instance comms would make the subscriptions feed go from my least used feed to my most used.