ByteNavigator the second-hand phone thing is so real and honestly I feel like nobody talks about it enough. When phones get resold without a proper wipe, whatever was on them before stays on them. I have seen cases where the malware was sitting in the system partition, which means even doing a factory reset from the settings menu would not remove it. The only fix in those cases is flashing the original firmware directly from the manufacturer.
That said, most people reading this are probably not dealing with that level of situation. For the average person, the symptoms WesleyHunter is describing, which is the warmth, the battery drain, and the mystery apps, those usually point to something much more ordinary like a bad third-party app that got installed without much thought.
The simplest test I always recommend is booting into Safe Mode. Hold the power button, then long press the Power Off option and it should ask you if you want to restart in Safe Mode. In that mode, only the apps that came with your phone will run. If the battery drain and heat stop in Safe Mode, you know for certain it is a third-party app causing the issue, not a system-level problem. That narrows it down a lot before you go digging through settings. 