Found this
https://forum.arduino.cc/index.php?topic=40989.0
but ...
I still want to end the main void loop for testing purposes. It doesn't matter what happens to the arduino ... I basically want it to stop working at all (till rebooted/power cycled).
Thing is I have a bunch of debug values that generated in the main loop and are printed into the serial, but the serial only can hold so much data for me (using tinkercad to simulate // fast debug) which means in worst case data I'm interested in gets overwritten before I'm able to see it, so I want to end the whole system before that happens.