I think in addition to all the other replies, a key in how this happens on more of a physics level is how spread spectrum communications work. In short, it’s a technique to ‘squish’ a high-amplitude, narrow bandwidth signal down to a low-amplitude, wide bandwidth signal. The resulting low-amplitude signal can be lower than the background noise. Look at Fig. 6 here: https://www.maximintegrated.com/en/design/technical-document...