Does that mean what I see in All is a combination of instances my instances is federated with + the comms that users from my instance are subscribed to? So if my instance is federated with another instance and no one from my instance is subbed to any of the comms from that other instance then no one from my instance will see posts from that instance in All?

Sorry for word salad.

Do you mean instances can federate with communities and not just other instances?

Someone will correct me if I’m wrong but I think that you will see more stuff in All if more people from your instance are subscribed to more communities from other instances.