It is what is called a "blastr" service. It will find any events you have authored on any of the relays within its list of 40 or so relays, and it will then broadcast your notes to that same list of 40 relays, so all of your notes will be on all 40 relays, assuming they are online and accept them. Since these are relatively popular relays, chances are that most people are reading from at least one of those 40, and Primal certainly aggregates from several of them.