Why my iPhone XR battery health drains when playing Call of Duty mobile game?

You need to replace your battery, Apple recommends replacing it when it reaches 80%, yours is overdue. The iphone XR was discontinued in Sept 2021, almost 3 1/2 years ago, yours could be older. The battery is at the end of its life.

iPhone Battery Replacement.

iPhone Battery Replacement - Apple Support

Your battery health will decrease anytime you use the phone. The battery is a consumable part of the phone and as it is used, recharged, and used again, battery health will decrease. What is your current Battery Health percentage? How long have you had the phone, and when looking in the battery area, what is the number of charge cycles you have used?

Kurtosis12 has provided you with the best answer, and a link to set up an appointment.