https://wccftech.com/ios-15-lets-you-turn-off-automatic-night-mode-switching-on-iphone-heres-how/

Apple's Night Mode on its latest iPhone models is great as it intelligently lights up the environment with software. If you are in a darker environment, your iPhone will automatically shift to the Dark Mode and there is no toggle to switch it off. To be fair, there have been countless times when I personally wanted to take a darker aesthetic shot but the Night Mode got in the way. However, this is about to change with iOS 15 as you will be able to manually turn Night Mode off on your iPhone and keep it off.

While you have the option available to turn Night Mode off on your iPhone, the moment you close the app and open it again, it would reappear as enabled. Your iPhone's camera basically detects low light and then automatically enables the mode. If you are tired of this automatic behavior, iOS 15 will allow you to completely disable the feature (via 9to5Mac).