Phone will not turn on.

So my friend gave me his One Plus 8T which one day all of the sudden completely turned off during a call and has not been able to be turned on again. He discarded the phone as a lost cause when a phone repair shop told him that the phone wont turn on due to a fault motherboard which cost 400+ euro to replace, so I decided to maybe work out the issue as a fun little project.

I tried the various ways to reset the phone like holding the volume and power buttons for 20 seconds and tried to let the phone charge for multiple hours (with no indication on the screen). All of this without any result.

I would like to explore some ways to determine whether or not there is a hardware issue, but I am inexperienced when it comes to this kind of troubleshooting.

Any suggestions of things I can try or check out are appreciated and if more details are necessary, just ask!