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.
macOS Catalina Now Available
Apple Releases iOS 12.3.2 With Portrait Mode Fix on iPhone 8 Plus
The iOS 12.3.2 update is available on all eligible devices over-the-air in the Settings app. To access the update, go to Settings > General > Software Update. As with all iOS updates, iOS 12.3.2 is free to download, but it may take some time for iOS 12.3.2 to propagate to all iOS users. Today's update is limited to the iPhone 8 Plus and it is not available on other devices.
According to Apple's release notes, the iOS 12.3.2 update addresses an issue that caused the Camera app to capture Portrait mode photos without a depth effect on the iPhone 8 Plus.
Apple is also working on an iOS 12.4 update that's set to be released in the near future, as well as iOS 13, which will come out this September.