Heating up, discharging super fast and won't charge after battery dead

Hello, I had a problem with an iPhone 6S which was draining super fast even when idle (background refresh and all apps turned off). I though it might be a battery, so I replaced it for a brand new but the problem persisted. I noticed that phone was heating up near back camera, so I tried to plug off flex cables one after another on that side so I could tell which one gone bad. But it was neither of them, even when everything was plugged of, that side of iphone was still heating up and battery drained so fast. That battery is dead the phone doesn't even react to charging from a wall charger (other iphones do, so it's not the charger). I put back the original battery after a while and it magically charged to 65% and not more, then I took it out of a charger and after a while it was dead. Then it would charge only on 5%. and that’s all, the phone is still heating up and discharging fast.

This iPhone has all the parts original, never was serviced until my battery change. It was not the first time I was doing it and nothing went wrong.

What could be the source of heating (and therefore super fast discharging I suppose) and why the phone won't turn on and react to charger? Thank you for your responses.