This is a more complex question on Lemmy than on Reddit.

On Reddit you have only one group (admins) in charge of the whole system, so you report a bad community to them and they ban it.

On Lemmy, there are many servers with their own admins. So for example, you made your account @lemm.ee but you’re posting this question to asklemmy@lemmy.ml. Lemm.ee and lemmy.ml are two totally different systems, run by different operators, with different rules and acceptable use policies. And these policies vary widely. Some instances allow or block NSFW type content (porn), some communities allow or block discussions of piracy, some communities like beehaw use more active moderation to try and have a more inclusive space and thus prohibit a lot of things they consider offensive. These decisions are up to each instance admin.

In general, if you want to report a community, you should first figure out if the community violates its own instance rules. You can find the instance rules and a list of admins to whom you can report it on that instance’s home page (https://instance.tld).