Apologies in advance if I am using the wrong terms or posting the wrong way.

Onto my question, I see that despite being on kbin.social, I see articles for lemmy.world and other platforms on this site. Did the admins of each instance configure this, or does it occur because they are all public and each instance fetches data from everywhere else?

The reason I'm curious is because I'm wondering about building a read-only instance for myself. Hosting an instance of kbin/lemmy/etc comes with a lot of extra functionality (users management, communities, content uploads, etc), along with associated costs (storage, scaling). I imagine having a instance that simply reads other federated instances, caches their posts/comments temporarily, with minimal storage for configuration (subscribed topics/people/etc).