Because all of the signals are superimposed. So if your receiver isn't selective it will show all of them at once and if you then demodulate selective parts of the spectrum by filtering you can isolate the signals individually.

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.

I have no clue, but I would guess that they do not have a single atom but rather an entire crystal of them.