My phone is overheating

It's not just when it's on, but even after i shut down the entire system. Battery died around an hour later even with the power completely off. Even with every single battery-saving precaution combined(airplane mode, wifi/bluetooth/hotspot off, low battery mode, all apps closed, lowest screen light settings it still eats battery like my last iphone(an iphone 7) while gaming. Is there any way to fix it? what would the repair price be like?