iPhone batteries degrade based on usage/ charging cycles.
Usually the battery will degrade at least 1% per month. However this can vary widely depending on how many times you you charge your device per month, and other factors.
A brand new iPhone may already have 1 or 2% degraded at purchase, and this is completely normal.
The more you use your device, the more you have to charge it, and therefore the faster it degrades. There sometimes can be an issue with either software or hardware that can cause batteries to drain or degrade at a high pace. But this is rare.
Usually it's something on the user end though. Not saying that's you, but that's usually the case. A game or program is usually the culprit, or something in the settings.
Toggle off things such as Wifi, Bluetooth, and Location Services when you are not using them. These features can drain your battery quickly, especially if they are constantly searching for a signal. Cellular drains your battery faster than anything, so when possible turn off cellular data and use wifi only.
Check for apps that are utilizing too much of the battery: Go to Settings / Battery and view which apps are using the most battery. If you notice any apps that are using a lot of battery in the background, you can try disabling their background activity or uninstalling them if you don't use them frequently.
Keep your device up to date on software for the best battery performance.
Keep your brightness as low as possible.
Keep in mind Apple will not replace a battery unless it drops below 80%.
iPhone Battery and Performance - Apple Support
About the battery and performance of iPhone 11 and later - Apple Support
Check the iPhone battery health and usage - Apple Support
If your iPhone or iPad is running slow - Apple Support
Charge the iPhone battery - Apple Support
