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 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.

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