I have had this issue with my Pixel 4a 5g. After trying everything this and other articles are suggesting I took it to a certified technician who found no HW fault, but the phone still didn't work. They I talked to another technician, who said this is a typical Pixel issue caused by the processor. The processor is very likely lost its firm connection with the motherboard, due to a drop, wet conditions or heat. I couldn't recall any of these happening, but the thing still turns on and off. It is virtually non-reparable. As I understand it this is an issue with the production. My phone is just out of warranty... So, now this is a great donor phone for someone, who doesn't need the motherboard. Sad story for the Pixel.