subreddits are all part of reddit, there's a top part that can decide over all subreddits and make rules and ban people etc. Lemmy does not have a central point of authority. lemmy.world can only make rules and control lemmy.world, lemm.ee can only decide over lemm.ee. If you want your own rules, you can make your own instance and be as valid and part of lemmy as any other instance. The main point is: there is no level above this that controls all instances. Each instance is the top level of authority for that instance, and anyone can create an instance if they have the knowledge and resources.
Another aspect is that technically you could also interact with mastodon, peertube, etc, but that isn't seamless and there's no consensus if it's even a good idea to pursue that, but it's technically possible.