How does the external IMU work?

Can anyone explain the working principle of external IMU? Or provide relevant research information, because I am a newbie and have not been able to find more critical relevant information.
There are mainly the following issues:
According to the setting instructions on the ardupilot official website (External AHRS Systems ā€” Copter documentation)
There is no mention of turning off the internal IMU. Why is there no need to turn off the internal IMU? Is it because the flight control will force the use of an external IMU? Or will it integrate external and internal data?

Thanks for the advice

It will create different EKF lanes, one for each active IMU. Disable the internal IMUs manually if you do not want them

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Sorry, Iā€™m not sure how to close it manually, but I found the following parameters. Can I close it through these parameters?

INS_USE2 = 0
INS_USE3 = 0
INS4_USE = 0
INS5_USE = 0

Iā€™m sorry if I got the parameters wrong, please give me some advice.

Yes, those are the parameters that you should change.