Short battery life on iPhone 16 Pro Max

If it's brand brand new, for the first few days, it's busy indexing millions of bytes of data, while you're likely using at the same time. Give it some time. You can also see what is using the battery, by Opening the Settings App, tap Battery and you'll see what is using your battery the most.

Finally, read this --> If the battery in your iPhone drains too quickly - Apple Support

Let’s see what apps are the largest percentage of your battery’s daily and weekly capacity. Please follow this path,

iPhone > Settings > Battery > please take screenshots of the battery usage by app for 24 hours and 10 days and share them here,

How good is your cellular connection and what do you have it set to on your iPhone? Settings App > Cellular > Cellular Data Options > It should be set to 5G Auto or, if 5G is not strong in your area, set it to LTE. If your phone is working hard to stay connected to what you have it set to be connected to, it can use a LOT of power.