The power on / off switch is ok. It just does not power. It would be helpful if you could get the schematic diagram of the device.
I do not see any signs of internal heating.
Battery voltage is 3.5v
Where can I start?
Not even when I put the tablet through the USB cable, Windows does not detect the device

I tried to use this program (Faheem Anjum Android Tablet Tools) without success, it does not recognize the device

Charge battery directly from external power supply to 3.7V minimum, 4.2V - fully charged.
Then you'll try power on again.