The same thing that has been explained on Android so far applies to the iPhone. According to Apple's homepage, if you repeat full charge to discharge 500 times, the battery capacity can only be charged to 80% of the original capacity. If the situation does not change after trying all of the following ways to improve the battery depletion, it is possible that the battery has reached the end of its life.
How to make your iPhone battery last longer
.Update to the latest version of iOS
.Remove from heat-prone cases to prevent heat effects
.Enable low power mode for iOS 9 or later
.Adjust screen brightness
.Turn off push emails
.Turn off push notifications for your app
.Turn off location services
.Turn off Wi-Fi
.Turn off bluetooth
.Turn on "Reduce parallax effect"
.Turn off background updates
.Charge about 50% before long-term storage
If you've seen how to improve Android, you'll find that most of the improvements are the same for the iPhone. Please check Apple's official page for details on the settings and procedures for how to make the iPhone battery last longer.
As with Android, some services cannot be used by turning off the function. Please be careful when changing the settings. It is effective to be aware that each function is frequently turned off when it is not needed.