The physics
A transformer has two coils on one iron core. An AC in the primary coil makes a flux in the core. This flux changes all the time. The same flux passes through every turn of the secondary. So each turn of either coil gets the same emf.
Vs / Vp = Ns / Np
A steady DC makes a steady flux. A steady flux gives no emf. So the secondary gets only a kick at the switch.
An ideal transformer wastes nothing: the power in equals the power out. So when the voltage steps down, the current steps up. A real one gives out only the share η of its input.
Vp Ip = Vs Is Vs Is = η Vp Ip
A power line wastes I²R as heat. For the same power, a higher voltage needs less current, I = P / V. So the loss falls as 1/V². The line also drops a voltage I R. So less voltage reaches the town than the station sends.
loss = (P / V)² R Varrive = Vsent − I R