Within the Settings menu, there is an option to display Battery usage by various apps. This will give a hint as to what may be drawing power. Anything that relies on the GPS being active all the time, such as navigation software or fitness trackers, will suck quite a bit of power from the battery.
This should not be confused with the recent news reports about Apple deliberately slowing down the processor speed of phones with degrading batteries in order to help prolong the phone and battery life. Replacement batteries are readily available from Apple retail outlets and if a phone is more than a couple of years old, it is worth considering getting a new battery fitted. Cheaper sources of replacement batteries exist and you can do it yourself from kits available on line.
Bear in mind that you get what you pay for and there is a reasonable chance of bricking your phone if you opt for the diy method.
A visit to any Apple store and a chat with the staff can often resolve issues within a few minutes as they will generally check your phone and give you any information or tips you need to keep it running in peak condition free of charge.