Xbox Series X Motherboard
Connect 12V power up to the 12V connector pins via a bench power supply to first check for shorts. You should have less than 15mA power draw. Sometimes when first connecting it can pull 3-6A as the console auto-boots from a failed power up but if you wait it should settle down to milliamps. If it does not settle down and the power draw is solid and not bouncing up and down you likely have a short. Use a thermal camera to find the short.
If you have a daughterboard that gets to 1 second white light, then connect the daughterboard up to the motherboard and press the power button. Nothing much happens on the motherboard until you send a power on signal.
With the motherboard and daughterboard connected together when you power on, the white LED on the power board should now light up longer. If it lights up white for around 3-5 seconds then turns off, it is usually an indication the WiFi/Bluetooth boards are not connected.
IMPORTANT: If you don't connect the small brown Wifi/Bluetooth boards, the console will power on with white light for about 3-5 seconds then power off.
If the white light stays on, the console should be booting. If you see nothing on the HDMI output investigate the HDMI port and retimer chip near the port.