There is no „ground“ in that sense. For a current to flow, you have to create a connection between the source anode and kathode (e.g. + and - of a battery).

You can think of the defibrillator as a battery with a very small but powerful charge. You close the circle by touching both pads to the body. The current will flow through one pad into the body, through the heart and „restart“ it and from there into the other pad. No current leaves this system (=goes into „ground“), it goes back into the defibrillator where the loop is closed.

Why do you get a shock from touching a power cable then, you ask? Without touching another pole and thereby closing the circle? (Note: power cables are AC but for simplification, above DC example can be applied)

This is because for our power stations, the ground is acting as a pole, the current can flow through the ground back to it and thereby close the loop. Therefore, you can reduce the risk of a shock by using boots with thick rubber soles - the rubber acts as an insulator.

Btw, „ground“ is an often misunderstood term. Voltage is the difference between the electric potential of two points in a system. What is often called ground, is the common reference potential.

As powerlines use the actual ground as reference potential, „ground“ has become the defacto name for that - but it’s just a point of reference and can be of any potential. I could measure a voltage (=difference) between the ground of two different systems