Good luck! Hopefully it's something simple/cheap. If it still doesn't power on with the flex disconnected then the logic board is likely at fault although you could still dive deeper and disconnect components (SSD, speakers, display) to further isolate the logic board. To verify that the top case (it houses the power button) is at fault you should disconnect the top case flex cable (silver flex in the photo) and try again if the Mac doesn't first power on with the power pads. Something to keep in mind, a stuck power button will prevent a Mac from powering on even when the power pads are shorted. Source picture can be found in iFixit's 13' Mid-2011 MacBook Air logic board replacement guide, used and modified under a CC license. Be sure to use a metal tool with a non conductive handle to bridge the two points.

Bridging these two pads while power is provided to the Mac should power it on. See the circled pads in the picture below. The power pads, two slightly raised solder points, are found just above the keyboard flex connection on the logic board. LOT OF 2 X MACBOOK PRO A2251 2020 & A2485 LOGIC BOARDS FOR PARTS ONLY - BROKEN. I assume you're looking for a way to bypass the power button to power it on? If so this should help. APPLE MACBOOK PRO 13 RETINA A1502 2015 TOUCHPAD TRACKPAD FLEX CABLE.

Macbook pro 13 logic board power pads