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.
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[Update] - Additional countries as reported by MacRumors.com readers
[Update #2]The official list as of 9 a.m. PDT includes Austria, Belgium, Finland, Greece, Italy, Luxembourg, Netherlands, Portugal, and Spain