Why is my phone getting very hot?

Hi guys, I recently bought this Samsung S8 from someone who claimed it needed a new battery as it was dead. I could tell they weren't the tech savvy type so they obviously weren't trying to rip me off. I put the new battery in and it works great. I start going through factory reset setup and it gets very hot on the right hand side by the lock button. The iris scanner, finger print scanner and camera also do not work and refuse to open for some reason. This phone to my knowledge was never opened and appeared that way when I removed the back cover.

The main board looks perfect, no signs of water damage, the LDI sticker isn't activated so it's definitely not water damaged. It's just getting too hot to handle. I checked CPU temps and they reach 50 degrees just at idle. Usage is normal however on CPU and memory.

I have restarted the phone, cleared cache memory from camera app, wiped boot cache I believe when booted into recovery mode.

Any ideas would be great as I was really excited to upgrade :/

Thanks!