Others are already attempting to explain everything so I will try to just focus on solving your problem.
You are using the app Lift Off and you have linked your lemmy.world account. From the main screen click the icon in the top middle of your screen and select “ALL” under the lemmy.world section to see posts from all communities on instances that are federated with lemmy.world. That’s all you need to do to see posts from all the other instances.