How multiple data transmission (in the form electrical signals) work?
It's best to view this in the frequency domain rather than the time domain. 

Here is a basic example - two signals SIG (~1 MHz) and SIG2 (~2 MHz) are combined to form SIG+SIG2, then passed through LC filters with corresponding resonant frequencies to separate them again:

Schematic:



Signals in and combined:



Signals Out after filters:



FFT of SIG + SIG2, and FFT after each filter:



Note the difference between SIG+SIG2 in the frequency domain as opposed to the time domain (blue waveform in second plot) - the two separate frequencies are much easier to see.