If i understand your quesion, this is the answer.
Under light load, no boost, the ECU uses the O2 sensor
to determine fueling. There is not not much you can
do to change that. With black boxes, after market fueling
controllers, all the changes up made Are countered by the ecu.
Tuning a new chip may make a difference, but would not change until re-reprogrammed. It only aims for target A/F ratios.
Under boost or high load, the ecu switches to open loop
and adjusts fueling by maps, so no contol is exerted by the ECU.
Just predetermined fuel and timing points. This is where black
boxes can make a difference.
The ECU does not self "learn" per say.
Not familair with the 4AG, but it is probably similar to ours.
Regards;