You can see posts/conversations from other instances if the instances are federated(communicate with each other), which most are/do(?). You just have so subscribe to the communities that aren't on your instance to do so. But some instances aren't federated. For example, Beehaw defederated from lemmy.world (and another I forget which) early on due to the open sign ups and moderations issues. I'm on lemme.world, I can go on a beehaw community, I can see the posts, and read through the conversations, but I don't think they can see if I reply to anything( so I don't). But this is where I start to lose my grasp on the technology too, so if anyone more knowledgeable than me wants to chime in...