A transformer has two coils of wire wound around a shared magnetic core. The side connected to the source is the primary. The side connected to the load is the secondary. There is no direct wire between them. Power crosses the gap through the magnetic field in the core.
The ratio of turns sets everything. If the primary has twenty times as many turns as the secondary, the secondary voltage is about one twentieth of the primary voltage. Voltage scales with the turns ratio.
Current goes the other way. Because the transformer moves power rather than making or destroying it, stepping the voltage down raises the current the secondary can supply. That is the whole trick, and it is why the same device can sit at the top of the system and at the curb in front of a house.