The secondary. The phrase "step down" is understood to mean that it is the voltage that is being stepped down. Voltage being stepped down means that current is being stepped up. That is because, disregarding a small percentage of losses, the power is the same on both sides. But power is fundamentally the product of voltage times current. So if (V times I) on one side is the same at (V times I) on the other side, and if V is lower on one side, then I must be higher, in order for the product to be the same.
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