As pointed out in the answer by gnasher, a packet is nothing more than a sequence of bits.
The actual transmission depends on the underlying physical medium. In case of wireless networks, we transmit using radio signals or microwave signals or infrared signals.
How do we send information via signals?
We modulate the wave. We take the initial signal, which is just a wave (and contains no data/information) and then somehow combine our information signal with the base signal. This process is called modulation. The result is a signal that can be transmitted via airwaves.
Note that wireless communication is omni-directional. They are propagated in all directions. At the other end there is a receiver which listens for certain waves (say only listens to certain frequencies). The receiver knows which modulation technique was used to combine the carrier(base) signal with the information, and uses an appropriate technique to demodulate the signal to extract the information.
In case of digital signals (sending 0's and 1's), this is known as Discrete Modulation.
How do we distinguish a packet in a signal?
Till now we have established how a stream of bits can be propagated wirelessly. Distinguishing Layer 2 frames / Layer 3 packets / Layer 4 segments is a job for the Router / Computer which uses the receiver.
A simple example.
Say each link-layer frame starts and ends with one zero and six 1's. So the receiver which is constantly listening for airwaves with a certain characteristic (amplitude / frequency / phase / whatever) suddenly encounters a string of bits "0111111101101.....". This is immediately processed as the start of a frame and we keep on reading the signal until we encounter ""....101010111111..."". This is how we know that the frame has ended.
Now note that in the above explanation we skipped over a huge amount of topics. There are multiple digital modulation and demodulation techniques, as well as dozens of protocols that a frame / packet has to follow to be correctly processed by the target even if it is received. The Wikipedia page for Modulation will be a good starting point for further reading.