HITL sim and returned sensor values

Bringing up a HITL/offboard/pixracer environment. Have got it running, and tried a simple flight in the sim. Everything connects - sim, rc, drone, pi 3, qgc. arms from radio, and takes off with throttle. and then exhibits uncontrolled yaw - spinning in both directions, under modest influence from tx.

Of course, PID could be wrong for yaw - though it is fine in real flight.

But, it made me realize I have no idea what the sim does with the sensor values returned from a quad that is just sitting there. Can anyone fill me in, or point me to a useful resource.