Can anyone elaborate on this? How does a single receiver produce multiple concurrent outputs, and how are they isolated in this context?
Think of any antenna: it is just a rod or a coil, it may have a specific frequency that it particularly likes because that is a nice fraction of its wavelength or close to its own resonance frequency, but that doesn't mean it isn't going to receive all the other signals to greater or lesser extent as well. The ratio between that one that it likes and the rest is called selectivity. The lower the selectivity the more evenly you will receive all signals at the same time.
Usually receivers have a tuned front-end to get as much of the signal you want and to repress the rest as much as possible but that is optional, you can have a wideband front end just the same.
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.