Many on the net have been reporting iPhones getting rather hot when the YouTube app is running,
and yes the fine folks over at YouTube iOS team has gotten wind of the ‘ complaints,, and “are actively working to fix this. So they are acknowledging the problem, but is still unknown if the fault is with Google’s code or with iOS itself.
While waiting to update to the latest version of the YouTube app, you can resort to watching vids via the browser in the meantime, which doesn’t seem to cause such problems.
Hi Shivam, we're actively working to fix this! Thanks for your patience.
— TeamYouTube (@TeamYouTube) November 13, 2017