As per Apple’s website, Apple has revealed that the new WiFi hotspot feature in iOS 13 will allow devices to automatically connect to your iPhone when it detects that there is no internet connection nearby. “Automatically connect to your nearby iPhone personal hotspot when there’s no internet connection available.”
Presumably this automatic connection will require devices to have connected to your iPhone before or there will be some kind of authentication, otherwise it would result in all devices connecting to it and draining your data. Apple has also announced the inclusion of a persistent person hotspot feature.
According to Apple, “Remain connected to your nearby iPhone personal hotspot even when your device sleeps, so you can receive incoming messages and push notifications.” This is some of the changes that Apple will be making to iOS, where they seem to be making the platform “smarter”, like they have done with the battery charging system in a bid to help preserve battery health of your iPhone.