Night mode is an automatic setting which takes advantage of the new wide-angle camera that's in the iPhone 11 and 11 Pro models. It's equipped with a larger sensor that is able to let in more light, allowing for brighter photos when the light is low.
16" MacBook Pro Now Available
Apple Releases Revised Version of iOS 13 and iPadOS Developer Beta 3
The reason for the new version isn't entirely clear, but the original third developer beta was not available for the iPhone 7 and 7 Plus for some reason. Today's update, build 17A5522g, is only a very minor increment from the 17A5522f build of the original release, but Apple appears to be pushing it out for all devices, not just the iPhone 7 and 7 Plus that had not originally received it.
Registered developers will need to download the profile for the iOS 13 and iPadOS betas from Apple's Developer Center. The revised Beta 3 can be downloaded over-the-air once the proper profile has been installed.