IPhone - 6s is not turning on

Hello,

My phone was charging and it was around 70% and suddenly the screen got black. I tried to switch on by hard reset by no luck.

Then I went to Apple authorized service center and got answer like it's hardware issue, motherboard has failed and they have suggested the option to replace the phone itself.

I wanted to check, is it possible to replace only failure hardware for iPhone 6s? If yes, how much does it cost?

What could be the root cause for this failure?

I live in Rajkot, Gujarat, India.

Please guide me here.