Which mixer input is varied or tuned to convert signals of different frequencies to an intermediate frequency (IF)?
Why In a superheterodyne receiver the mixer produces the sum and difference of the two frequencies fed into it, and the IF filter is built for one fixed frequency. Since the IF stage cannot move, the local oscillator is tuned so that the difference between it and the desired incoming signal always equals that fixed IF. Tuning the radio across the band is really just tuning the local oscillator: LO = signal frequency plus or minus the IF.
Watch out The RF input is whatever the antenna delivers, so you do not vary it to select a station; the beat frequency oscillator works later in the chain to make CW and SSB audible, and the image frequency is an unwanted response, not an input you tune.
Fixed IF, moving LO: turning the dial turns the local oscillator, not the IF.