If you are talking of the simulation API by themselves (so you are developing a controller simulation), SendSignal will not send a signal directly to the robot but to the PLC. To send a signal to a robot, you need to use SendSignalToRobot method in this same class.
SendSignal makes sense only in LineSimulation. SendSignalToRobot will also work in Standard mode.
GetSignalValue will work in both modes, wherever the signal is coming from (from the PLC or directly from another robot). It will retrieve the value of a signal, it will not wait for anything.