The charging icon is constantly on in iPhone 15 Pro Max

Try a forced restart.

• Press and quickly release the volume up button

• Press and quickly release the volume down button

• Press and hold the side button.

• When the Apple logo appears, release the button.

Force restart iPhone - Apple Support (IN)

This usually means the port is dirty. Use a toothpick and a flashlight and get any lint and debris out of the charging port.

Then try cleaning out the port as Zachyy suggests.