The documentation describes the state vector and the state transition function. The insfilterAsync uses a continuous time motion model and discrete time measurements. So the state update equation is dx/dt = f(x(t)). In the documentation you can see that the position is elements 8:10 of the state vector. The derivate is the velocity which are elements 8:10 of the f(x(t)) function.
The measurement equations are not in the documentation. To see the equations, for the magnetometer fusion, in the code, type