battery drain

I have a new iPhone 11 Pro Max and find that the battery drains from 100% to 20% in 3 hours and the phone runs very hot, to the extent that if I leave the phone in my pocket it comes up with a high temperature warning and shuts down.

If I turn the phone off the battery continues to drain.

Despite being on to Apple seven times, totaling over 8 hours, I have no resolution to the problem.

Not good from a supposedly 'premium' brand!!!