Why my phone battery quick dying

As a battery ages, the chemical reaction it undergoes to charge and discharge gradually stops working, resulting in shorter and shorter battery life. Your phone is about six years old now, and if your battery hasn't been replaced before, it is almost certainly ready to be replaced now. That will take care of the shortened battery life and make your phone charge like new again.

If you go to the Battery page under the Settings app, you can view the battery's health information to see what the phone thinks of the condition of the battery. Generally anything below about 80% is considered used up and should be replaced.

If you're interested in replacing the battery yourself, iFixit has a guide giving you step by step instructions on how to do the job.

iPhone 11 Battery Replacement - iFixit Repair Guide

Note that Apple started pairing the battery's BMS (Battery Management System - the little circuit board attached to the battery cell's positive and negative terminals) to the motherboard, so unless you pay Apple to replace the battery you will get a pop-up notification for a week or so telling you that the phone cannot determine if the battery is a genuine Apple part - even if it is. There will also be a permanent badge notification on your Settings icon with a note to the same effect on the battery setting page. In addition, you will no longer be able to view the battery health information.