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.
Small differences in sensitivity have little, if any, effect on what you can or can't hear. Increased sensitivity means hearing is better, not getting more stations. A reciever can only process the signal it gats from an antenna. A reciever with average selectivity connected to a good antenna will probably outforform a reciever with good sensitivity connected to an average antenna. Miscellaneous A shortwave radio can also come with a noise limiter or a noise blanker.