It sounds that your electricity supply is a three wire single phase system which is also called a three wire split phase system.
It is probably not a three phase supply.
There are three conductors of which two are live (hot) and often colour coded black and red and a third called the neutral which is colour coded white.
There may well be a fourth wire with no insulation on it or with green or green and yellow insulation which is the earth.
The voltage between a live and neutral is $120 \rm V$ and the voltage between the two live wires is $240 \rm V$.
The wiring diagram for the hostel might look something like this.
You will note that each live wire is protected with a circuit breaker (CB1 and CB2) and the comparatively low power consumption $120 \rm V$ devices are connected between a live and the neutral and high power consumption $240 \rm V$ devices are connected between the two lives.
Suppose that circuit breaker 2 disconnected live 2 (L2) then fan 2 and the air conditioner would not work whilst the charger and fan 1 would continue to function.
The alternating voltages are as follows and have a frequency of 60 hertz (Hz).
In fact live 1 voltage is $180^\circ$ out of phase with live 2.
This is achieved by the use of a centre tapped transformer which you may have seen at the top of poles?
For historical reasons North America settle on a $120 \rm V$ standard but that was found to be wanting when using devices which consumed a lot of electrical power and hence demanded large currents which in turn required thick copper conductors.
Increasing the voltage to $240 \rm V$ decreased the required current and hence allowed the use of thinner (and cheaper) conductors.
The system also has the advantage of reducing the current in the neutral when $120 \rm V$ devices are connected.
This is because the current through the neutral due to a device connected to live 1 is $180^\circ$ out of phase to the current through the neutral due to a device connected to live 2.
In theory if the currents through those two devices were the same no current would flow through the neutral wire.
The three phase supply to which you refer is used to convey large amount of electrical power and the voltage graphs for the four conductor (three live and a neutral) look like this.
Again if each of the three phases draw the same current the current in the neutral will be zero.
That is generally not the case but it does mean that the neutral conductor can be considerably thinner than the live conductors.
Transmission towers (pylons) usually carry conductors in groups of three with extra earthing conductors (for lightning protection) as can be seen below.
You question about impedance is all to do with the flow of electrical power from one device to another.
If you want maximum power transfer between the source (eg iPhone) and the headphones the best that you can do is to have the output "impedance" of the device equal to the impedance of the headphones.
Making the impedance of the headphones larger will reduce the power being transferred but may alter other characteristics.
However this is rally from a different era and with modern electronics impedance matching is not always necessary and having a high impedance set of headphones can potentially improve fidelity but you need to refer to the experts for this.