Fediverse is the whole universe of different systems communicating with each other in one big network, not just nsfw. It is made of instances (=servers =sites) each hosting a selection of communities (=forums) with topics. For exemple here you are on the fedinsfw.app instance, and inside the asknsfw community.

You can subscribe to any community on this instance or any other instance communicating with it, from the list of communities, a quick search, and many other ways.

While not as populated as reddit for example, it is far from a quiet place, most posts will get some form of interaction