Hello James
The answer is that both PoE and data are being sent over the same pairs of wires at the same time. This can be done due to the nature of the way PoE sends power, and the nature of the encoded bits on the wire.
PoE transmits DC power to devices by maintaining a constant voltage across the wire pairs over time. Conversely, data signals cause variations in the voltage across the wires. Due to these distinct characteristics, it is possible to transmit both power and data over the same pair of wires and separate them at the receiving end. Upon receiving the signal at the destination, one can “subtract” the voltage level provided by the PoE to obtain the original data signal.
There are 2^48 possible MAC addresses which is about 281 trillion. This number is considered enough to cover the current and foreseeable future of networked devices. If vendors adhere to the allocation and assignment rules set by the Institute of Electrical and Electronics Engineers (IEEE), then there is no concern about MAC address exhaustion.
Vendors like Cisco for example, may have multiple OUIs assigned to them, where each OUI delivers a set of 2^24 or over 16.7 million MAC addresses. There are also 2^24 possible OUIs and so far, about 30,000 have been assigned. So there are MANY more OUIs still available that can be assigned without the fear of running out of MACs.
I hope this has been helpful!
Laz