SWL offers many things worth exploring. The pictures above are shortwave radios one is a portable shortwave and the other is a full size SW radio. All recievers today are superheterodyne. This means the recieved frequency is changed to another fixed frequency before the signal is transformed into an audio signal you can hear. The mixer combines the signals from the local oscillator.
Intermediate frequency remains constant.
The mixer is normally followed by an intermediate frequency and feeds it to the detector. The detector is the reciever section that converts the intermediate frequency into understandable audio. If a signal is a CB or SSB, a beat frequency oscillator supplies a replacement carrier to make the signal intelligible. The audio amplifier stage amplifies the audio from the detector so you can hear it through speaker or headphones. Selectivity is how well reciever rejects other signals.