Don't feel bad, mate. There are a lot of niggly little details like that in this hobby. You get used to it. A sensored ESC can run a sensorless motor, but a sensorless ESC cannot run a sensored motor. The ESC needs to be able to detect where the rotor is in its rotation in order to fire the stator coils with the right timing to turn the rotor. A sensored motor has a sensor built into it and a wire harness going to the ESC for this purpose. This method gives you a smoother throttle feel at low RPM compared to a sensorless system. In a sensorless system, the ESC uses the feedback it sees on the motor wires (the back-voltage) to sense the rotor position and a "start up routine" in the firmware to get the motor turning, then switches to run mode, all in milliseconds. How smooth that start up routine is depends on the manufacturer's firmware.