Need help regarding the heating of the phone

I live in Mumbai,India and Mumbai heatwaves are crazy throughout the year specially in summer. My phone heats up without using it I tried keeping it in my bag whenever I went out. I tried keeping it close to my bottle but the water itself becomes warm within few minutes when you are outside. I am pretty sure overheating damages the battery, therefore I need some advice from you all on

how to keep the phone cool or maybe atleast at a normal temperature?

And how do I protect the life of the battery in conditions like this?

How can I cool my phone after reaching home?

Smallest of the smallest help will be appreciated. (I've tried using different types of phone covers they all turned out to be useless.)