Are you using the “dark mode” introduced in iOS 13? The function of switching the screen color scheme to black to make it easier to see even in dark places, but models with an organic EL display (iPhone X / XS / XS Max / 11 Pro / 11 Pro Max) can also save power. Organic EL displays have a structure in which each pixel emits light, and there is no need to emit light if it is black.
In other words, if you use dark mode frequently regardless of the visibility of the screen, the model with OLED display will have better battery life. There is no need to switch to dark mode when you notice it, and quickly return to normal mode when brightness is needed.
Dark mode can be activated by selecting "Dark" on the "Settings" → "Screen display and brightness" screen. This screen also has a function to automatically switch between light (normal mode) and dark depending on the time of day, so it is convenient when using day or night as the basis for switching between use.
You can also use the control center buttons. Since it is not displayed by default, open the screen in the order of "Settings" → "Control Center" → "Customize Controls" and register the "Dark Mode" button. Now you can turn dark mode on and off with the tap of a button on the control center. If you do not want to add a button, press and hold the brightness adjustment bar, you can also turn on / off the screen that appears.
There is another way to use Siri. Activate Siri and say "Dark mode on (off)" or say "Dark on (off)" for a short time and the screen will respond immediately. This method may be useful when you can't release your hand while driving a car.
Easy explanation of operation procedure