HOW DO I REMOVE AN UNUSUAL WHITE STRIP AT THE BOTTOM OF MY IPHONE 13 PRO MAX?

As descriibed in Line at bottom of screen:

"This bar is meant to indicate where the bottom of your screen is since there is no Home Button. It is normal and is a feature of the iPhone. At this time, there is not a setting to remove the line since it allows users to swipe and close the screen."

If you are referring to the thicker line where I described, and not the thin short line at the bottom center of the screen and it does not appear in a screenshot, then it is more than likely hardware. I suggest that you use the Get Support link at the top of the page here to contact Apple Support. Or, if you have an Apple Store close by, make a Genius Bar appointment to have the device examined by Apple. They will explain your options. Genius Bar Reservation and Apple Support Options - Apple

If you are referring to the thicker white bar that is at the level of the Flashlight and Camera icons, that would appear to be a part of the wallpaper. Try changing to another wallpaper image and see if that bar is still there.