We’re still weeks away from the release of iOS 13, and Apple has started beta testing iOS 13.1. It’s a highly unusual move; we can’t recall Apple ever starting external beta tests of its iOS point releases well before the the prior version is finished.
This first major update to iOS 13 will bring several features that appear not to be in the initial iOS 13 release. The early timing makes us think that iOS 13.1 will be release relatively quickly after iOS 13, perhaps even by late September when the new iPhones are expected to release.
Updated 09/18/19: The fourth beta of iOS 13.1 has been released for both developers and public beta testers.