i went through this boot loop drama twice. one time it was in a boot loop emergency call cycle and called 911 several times. i could not stop it or turn it off. i put it in a faraday cage (martini shaker) and let it die over night. then removed the case and restarted. that worked. i think the case was slightly pressing the power btn. case replaced and phone was alright for months.
2nd time. i notice phone is boot looping and getting hot on charger. bottom line is i can't stop it from looping. it's constantly looping and getting hot. thinking the battery is very low all this time. finally i put it in the fridge for about an hour. some say freezer. i was not that brave. when i get it back out it's completely dead. i wait for it to warm to room temp, gently charge it on usb (so slow). it goes into loop. then put on fast charger and success. i notice that the battery was super low. so maybe the looping was keeping it at 0 charge?
my take away is the power button has a problem. right now battery has full charge and lasted overnight. seems back to normal. so i've avoided using the power btn until i get a new phone or repair. let it screen time out and then use the finger sensor to turn on.
and i see no power button repair here....bummer.