The earth is a pretty good conductor and it is huge, so it makes a good return path for electrons. The 3-phase power leaves the generator and enters a transmission substation at the power plant. Typical voltages for long distance transmission are in the 155,000 to 765,000 volt range in order to reduce line losses. A typical maximum transmission distance is about 300 miles. All power towers always have three wires for the three phases.
Many towers have extra wires running along the tops of the towers.
These are ground wires and are there primarily in an attempt to attract Lightning . For power to be useful in a home or business, it comes off the transmission grid and is stepped-down to the distribution grid. This may happen in several phases. It often has circuit breakers and switches so that the substation can be disconnected from the transmission grid or separatedistribution lines can be disconnected from the substation when necessary. In many suburban neighborhoods, the distribution lines are underground and there are green transformer boxes at every house or two.