It *does* need power to transmit data, but it doesn’t carry it with it. The reader supplies it.
The reader emits a radio signal which is picked up by a coil of wire in the NFC card or device, acting as an antenna. The signal in the antenna is electricity and it is this which powers the chip that does some calculations on a code sent to it, changing it in a way only the chip should be able to and sending it back out another antenna. The reader then picks up this response and based on its contents knows which card it is talking to.
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