In the USA, at most (but not all) transformers, the center tap of the transformer is is grounded, so that at the final step down transformer, and at a household, the neutral ends up +/- 10 volts of earth ground. At my house, it's close, about .3 to .5 volts, but it's not zero. The final step down transformer only has 2 lines going into it, 6900 volts ac, and steps the voltage down to 220 volts, with a non-grounded center tap to provide a neutral. At my house hold, there is a grounded rod used for the third round pin socket on 3 pin plug sockets. The wide pin socket hole is neutral, and the narrow pin socket plug hole is one of the two end taps from the transformer, to provide 110 volts. A 220 volt socket would use both ends of the transformer, plus the grounded line.
There's a huge potential beteen helicopter and the line. The first part of the procedure is to extend a probe connected to the helicopter and the line being worked on, so that both helicopter and the line end up at the same potential, You often see significant arcing during the connection process. The line is then connected to the helicopter while the worker moves onto the line, then the line is disconnected. This processis repeated again when the worker leaves the line. The worker also uses a faraday cage like suit so any current flow or transformer effects of working on a single wire go thorugh the suit and not the worker. Example video:
There are also line inspector robots a bit less than 1 meter long that rely on a transformer effect from the two points on a single line to power them so they can move on high power, high tension lines. (I couldn't find a video of these in action). Just the field from these lines is strong enough to light up flourescent bulbs: