You can choose to show “Subscribed” communities—only the ones you’ve chosen, “Local” communities—only the ones on Lemmy.World, or “All”—which will pull from all the communities federated with Lemmy.World.

So to answer the question, posts from outside your home instance will show up in your feed, should you choose for them to.

Seeing as the vast, vast majority of mobile users are using the official app, it really doesn’t suck that much for them. I wish it wasn’t the case, but it is.