Does the ECU use the auxiliary battery during istop?

Does the ECU use the auxiliary battery during istop?
Meaning when the engine turns off & the a/c etc is then run from the auxiliary battery during istop.

I've been trying to figure out why my abs, tracking, hand brakes mil (warning lights are on) for my 2011 Mazda 5/ Nissan lafesta.
The fault codes from the scanner are many, but include low voltage supply, resetting the abs module worked (so far) after I charged the aux battery on repair mode for a day (need to purchase a battery diagnoser).

My thinking is the car engine stops (istop) then all electrical including the ecu draw only from the aux battery, so if the aux battery is failing then the ecu turns on the mil abs etc fault lights.
Is this the case?
 
done ~300km so far with no problem. My repaired aux batt tested ok 99%, resistance was twice my newish main batt.

Still know idea if the ecu relies on Aux batt???
 

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