As @tech_ni mentioned, it would be worth your time to try disconnecting the battery. My niece brought her Pixel 4 to me when it looked dead; like yours, no signs of life whatsoever. I opened it up, disconnected the battery and spent the next half hour checking everything I could. No luck finding the problem, so imagine my surprise when I plugged the battery back in and it booted up like nothing was ever wrong with it!
Still have no explanation, but it appears the simple act of unplugging the battery brought it back to life; that's literally all I did. Worth a try.