Why does is a motor's ground different to its power source?

I was learning about servo motors and I viewed a tutorial where the guy uses a 6v battery to power a servo motor while it gets its signals from the arduino nano. The ground of the servo was connected to the ground of the arduino instead of the battery. Wouldnt this damage the battery like there would be a shortage of electrons in the negative side of it or too much electrons in the ground of the arduino? How does putting a positive terminal of a battery to a negative ground 0v of another source work?

Hand-waving, no links, no schematics. Not useful.

AWOL: Hand-waving, no links, no schematics. Not useful.

Heres the tutorial https://www.youtube.com/watch?v=ybV8vitYAWU

3ngineer:
I was learning about servo motors and I viewed a tutorial where the guy uses a 6v battery to power a servo motor while it gets its signals from the arduino nano. The ground of the servo was connected to the ground of the arduino instead of the battery. Wouldnt this damage the battery like there would be a shortage of electrons in the negative side of it or too much electrons in the ground of the arduino?
How does putting a positive terminal of a battery to a negative ground 0v of another source work?

So the battery negative lead is not connected?

Normally you connect servo power source to its power and ground wires, and the controlling
device to ground and signal. There is a power circuit and a signal circuit, and they share ground.

yes, and that is exactly what he does in this video :)

@3ngineer : the nano takes its power from the usb - And he added a wire from battery 0V to nano GND, so that the nano and the servo share ground, it's OK

You always connect the grounds of multiple power sources when you're using them in the same project (except in unusual cases). The concept of "voltage" is meaningless without a reference point, and we use ground as that reference.

Without a common ground, the arduino could not control the servo - the servo would not see any signal, as the signal is transmitted by changing the voltage on the wire to the servo with respect to ground.

3ngineer:
Heres the tutorial
How to Arduino #3 - Servos - YouTube

It clearly shows the servo connected to the power rails on the breadboard which are powered
from the battery pack.

There is also a lead connecting the grounds.

You said:

The ground of the servo was connected to the ground of the arduino instead of the battery

which simply isn’t the case.