If you're just looking for the modern explanation of how an SDR works: an SDR just measures electrical current flow over time. If you graph current over time as

If you're just looking for the modern explanation of how an SDR works: an SDR just measures electrical current flow over time. If you graph current over time as detected it usually just looks like a bunch of signals all superimposed upon each other. A signal like "f = asin(x) + bsin(y) + c*sin(z)" if all the transmitters were just transmitting pure carriers with no modulation. Somewhere in the SDR, electrical current flow becomes a number using analog to digital conversion on the order of 7-12 bits in most SDRs. At that point you apply something like a FFT and then the time domain signal becomes a frequency domain signal, which can separate out different carriers.