If electric current is simply the flow of electrons where does it go? (say, in a power grid cable, not a battery) What is the starting and ending point?
AC is almost always used to describe a cable within a power grid. As you would expect, the current flows in a direction opposite to the flow of electrons. That’s what AC is. The electrons vibrate along the wires and go nowhere.
This vibration is actually very minute. Typically, each electron in the current moves less than one millionth of an metre from its source. This is true even within the power station generator. The generator’s electrons are located in nearly the exact same spot as when it was installed and in the same location as the copper wire before it was made into wire.
Answering the question, then, is that the beginning and end points are the locations where each electron was when the wire was made. AC electricity has no net effect on electron positions.
This just shows that while electric current can involve the flow of electrons but electricity is much more than this. The flow of energy is what electricity is about. This is not carried by either the electrons or the current. The electric field around the wires carries electrical energy, which isn’t a physical thing. The flow of electrical energy is always in the same direction, from the generator to load, along both wires.