Hey, areeba81.
I see that you're having problems with your iPhone 7 Plus. It's getting overly warm and the battery life has decreased. I'm happy to help.
Keeping iPhone, iPad, and iPod touch within acceptable operating temperatures -- This article provides the information and steps that you need to keep your iPhone within acceptable temperatures.
iPhone Battery and Performance -- This article not only explains how a lithium-ion battery behaves, but it also provides steps on extending the battery life of your device. There's also a new feature called Battery Health which will help manage performance. You can also see what's using the most power by checking the Battery Usage a bit lower in the app.
If this doesn't correct the problem, please describe the issue in greater detail. Let us know what percentage is the Maximum Capacity at. The more information you provide, the better we'll be able to offer additional guidance.
Take care.