What voltage is sent to the arduino GND when a transistor is used

hi

i learning transistor circuits following sources:

http://www.instructables.com/id/use-arduino-with-tip120-transistor-to-control-moto/
https://itp.nyu.edu/physcomp/labs/motors-and-transistors/using-a-transistor-to-control-high-current-loads-with-an-arduino/
https://learn.adafruit.com/adafruit-arduino-lesson-13-dc-motors/transistors

in 3 of examples emitter connected arduino gnd , wondering how current/ voltage sent arduino gnd pin?

thanks in advance

quote

i wondering how current/ voltage sent arduino gnd pin?

all of (current)

Arduino Forum > Using Arduino > General Electronics > What voltage is sent to the arduino GND when a transistor is used

arduino