Those are different individual values, not different communities. All of the conversation is co-mingled in the same threads that everyone sees. Everyone is expected to follow HN rules. 1 person can be a part of multiple communities.
An example of different communities would be subreddits. Reddit as a whole connects all of the communities with a front page, but each community is isolated from one another unless you choose to look over or someone else chooses to look into your community. Cross-community conversation happens from the overlap of subreddits followed between individuals.
That’s a silly definition. In real life everyone has access to the same street (feed) with the same shops/restaurants/bars/etc (posts), but people choose which of this they go to and they each have their own vibe. They don’t typically cross pollinate. So there are different “communities” within a “community”. Surely you don’t read and interact with every post, only those you are interested in.