Try uninstalling unwanted apps, you have various power saving modes, you set as per your need. Normally phone gets heat up when too much apps running in background........try full draining battery once in week and avoid overcharging.

same issue

try greenify from playstore and hibernate all apps except auto-launched apps...

same problem

I have not faced the problem yet. Phone gets hot it is normal but if it gets hot abnormally then you should go to your nearest service center.