Phone dies/wont turn on when kid playing game while charging

iphone will not turn on, no display, no connection, no heat after my kid brother play MOBA game while charging phone. I suspect something wrong with board and seems to be burnt hdd ?

Here is what I did:

1. check cable it charges my other iphone fine
2. battery resplacements done - no reaction, no heat from board either
3. screen replacement done with spare- still nothing display

While I cannot get the charging port to be replaced, but battery is independent source that rule out the power source issue. I am suspecting the charging while playing game gives too much heat and /or voltage ?that fries somewhere in the board but not very sure.

I personally done a lot of swap parts and really want to get into soldering. I do find many of the weird issue need more than just swapping and replacing. Having several spare parts/phone to work with, it will be great if someone is willing to give some clue on where to begin?