Starting Pi 4 with global_en

The Organization's schematic is incomplete, but it does show GLOBAL_EN connected to U2 - the PMIC (Power Management Integrated Circuit). Pulling GLOBAL_EN low (aka shorting to GND) resets the PMIC - which causes it to power up all its outputs, in turn causing the Pi 4 to boot.

Your question is not completely clear to me, but when you say, "I am not sure exactly how" you may be asking, "How would one implement this feature - shorting GLOBAL_EN to GND?" If that's part of your question, there are several ways this could be done, but I know of nothing in the current Pi 4 hardware that would support this. One typical way this is done on other machines is with a RTC (Real Time Clock), but Pis have never had one. Another way would be with an external sensor of some type that provided a switch closure when some condition was sensed. Still another way would be to use a small microcontroller (Arduino for example) to set the state of GLOBAL_EN.

The documentation states:

POWER_OFF_ON_HALT

If 1 and WAKE_ON_GPIO=0 then switch off all PMIC outputs in halt. This is lowest possible power state for halt but may cause problems with some HATs because 5V will still be on. GLOBAL_EN must be shorted to ground to boot.

Default: 0 Version: 2019-07-15

If you've not seen it, you might want to read this answer, or if you want full details try this GitHub page.. Now we've both confirmed shorting GLOBAL_EN to GND will in fact boot the RPi 4 from halt, poweroff or shutdown when the EEPROM firmware is configured iaw the documentation.