Does the Sirius module output the actual UBX message directly from the F9P or is there an onboard chip that convert it to Mavlink? In addition, for the I2C interface, is that directly connected to the magnetometer or the I2C of the F9P?
Yes, the Sirius module outputs the UBX messages directly from the F9P.
It is converted to MAVLink with the help of an autopilot that has the STM embedded.
For the I2C interface, it is connected to the magnetometer and the interface is interlinked to the F9P. Interfaces are configured via the configuration interfaces on the F9P.
I hope this helps.