Why A transformer uses two coils sharing a magnetic core, so AC in the primary induces a voltage in the secondary. The output voltage follows the turns ratio: fewer turns on the secondary than the primary gives a lower AC voltage, which is exactly what a step-down power supply transformer does with 120 V AC line power. Note that the output is still AC; nothing here converts it to DC.
Watch out A diode is tempting because it appears in every power supply, but it rectifies AC into pulsating DC rather than lowering an AC voltage. Capacitors and transistors do not change AC line voltage levels this way.
AC in, AC out at a new voltage means transformer. Change AC to DC means diode/rectifier.