You can see posts from every instance that your instance has federated with. For example, I, on toast.ooo, can see posts on lemmy.world, lemmy.dbzer0.com, and sh.itjust.works because my instance federates with them.
You can’t see posts from instances that your instance has defederated with, though, nor can you see posts from instances that have defederated with yours. Think of it like cutting one of those thick undersea cables that connect the internet across continents.
There’s a lot to consider when picking an instance; lemmy.world is a good default, so that’s probably why Voyager directed you to it, but don’t be afraid to switch to another instance of you think it’ll serve you better!