Since all the electronic components on the motherboard were all soldered. Therefore, no short connection will occur on it unless you accidentally made a short between them. The burnt mark found on the motherboard is mostly created by a spark from a loose connection.
And I think a short or fault connection from your computer came from the primary circuit of your power supply. Then the secondary circuit is automatically tripped off. So, no output voltage from your power supply is delivered to your motherboard.
You cannot test your motherboard without power supply installed. Charge to experience (sacrifice) or test your motherboard to a shop.
Okay, wait for some other ideas.