Zac Hall, 9to5Mac:

Apple has always allowed you to disable Night Mode for low-light shots, but the process isn’t fast. If the iPhone detects that Night Mode should be used, the camera automatically enables it.

Only then can you turn off Night Mode for that shot or shooting session. Fire up the camera after a break, and Night Mode will once again default to on when low light is detected.

And:

Turning this setting on tells your camera to keep Night Mode off if you’ve disabled it for a shot. You can still use Night Mode when you need it by manually engaging it in the Camera app, but you make the decision now instead of the iPhone.

Follow the headline link for the details on the new Night Mode default setting. Good to know that setting exists. More to the point, once you upgrade to iOS 15, worth spending time in the “Preserve Settings” section of the Camera settings to remind yourself what’s there.