Again, what is sd_app_evt_wait

[Paul]

I guess it puts the CPU into deep sleep mode and whatever interrupt will continue the code.

Is it possible to share any source code of this routine for better understanding what does sd_app_evt_wait do?

[Moloch]

It's a wrapper around the ARM instruction "WFE", which as you noted puts the CPU into a low-power state.

If I had to guess, I think Nordic has added some internal state-saving and safeties that keep the CPU awake and ready if some timing critical radio operation is about to occur.