Where do the forces exerted on flight yoke/stick comes from and how do they vary with flight conditions?
I've worked with control loading systems quite a bit. What you describe is a reversible system, where the flight crew feels the aerodynamic forces acting on the primary control surfaces (elevator, ailerons, rudder).




In the aircraft, the pilot pulls on the flying control, which is connected via a steel cable to the control surface, which then deflects. Airflow blows back on the deflected surface, with a force proportional to deflection of the surface. In the picture above $H_e$ is the hinge moment on the elevator, the aerodynamic part of which is $$H_{ea} = C_{he} \cdot ½ \cdot \rho \cdot {V_h}^2 \cdot S_e \cdot \bar{c_e}$$ with $C_{he}$ = moment coefficient, $\rho$ = air density, $V_h$ = airspeed at horizontal tail, $S_e$ = elevator area, $\bar{c_e}$ = mean aerodynamic chord of elevator.

The hinge moment coefficient is a function of trim tab deflection and angle of attack, and is measured in flight tests. Figure below shows $C_{he}$ for the Fokker F27. 

On ground, at airspeed zero, $H_{ea}$ is zero but there is contribution of friction, bob weight and down spring.





In the simulator, the same flight control is installed as in the aeroplane, but now is attached to an active actuator that is programmed to push back at the stick with the same force that would be experienced in the simulated flight state in the aeroplane. The actuator features an inner (control) loop and an outer (force feedback) loop.


Force vs. position and position vs. time measurements are taken from flight measurements, which must be tightly reproduced in the simulator in order for the sim to be certified. Reversible controls of small, simple aeroplanes are much harder to simulate than the irreversible, hydraulically actuated flight controls of larger planes, since these do not change as a function of flight state.