My understanding is that each instance has its own selection of content it allows to be hosted by that instance. But your username/password can log into any instance, and access that content. When you do this, the content is not transfered to the instance your UN/PW is from (your origin), but access to that content is granted as you have a federated log-in. What is brought to the origin instance is like a pin in a map that can then be seen by the origin server, and added to the origins directory for other users on the origin to see. Each instance has its own directory based on the pins that instances users have placed throughout the internet/fediverse.

Back to your question, and this is my understanding of how it works. You won’t find any NSFW communities hosted on sh.itjust.works, but you can see & access nsfw communities from others (lem.my, lemme.one etc…) using your sh.it login. I think you as a user will be able to add NSFW to your feed, but it won’t be on the directory for others to find if the admins blacklisted that content/community.

This is my first post on any of the fediverse, and I am partly posting this to see if I am way off base, so please correct me!