Will step through it..
Assuming 3.5 code... (3.6 should be able to use all hardware..)
Starting with IMU's
EKF3 runs a seperate EKF per IMU used.
The EKF switching is done based on the consistency of the data.
Compass, checks at preflight, so just pick your best.. I always use an external. The EKF will reject bad data, so multiple are useful here..
The Baro reports both, but uses the heated #1 as primary.
The cube has two internal barometers. Pull requests welcome to make better use of this.
Though the Baro does not do switching in flight, the data is very useful in the logs if you are having issues, as you can compare the two sources, and eliminate hardware as a cause of issues (or verify that the issue was hardware)
Ardupilot is open, so anyone wishing to join in on the conversation, and help drive code to be more robust, is welcome