Why does my phone a 6 get very hot when I use apps. Or the internet

Could be a number of things.

Try to close all the apps and see if that helps a bit. If not you can try to perform a Hard Reset by holding the home button and side button until your screen turns black and displays an Apple Logo.

If it's a hardware issue it could be your battery. You can have that checked out at your nearest Apple Store or at Licenced Apple Repair/Apple Premium Reseller.