Which mixer input is varied or tuned to convert signals of different frequencies to an intermediate frequency (IF)?

A superheterodyne receiver works by mixing an incoming radio frequency (RF) signal with a signal generated inside the radio to produce a constant intermediate frequency (IF). The internal signal is generated by a circuit called the local oscillator (LO). By tuning or varying the frequency of the local oscillator, the receiver can select different incoming RF signals and shift them all to the exact same fixed IF. This allows the radio to use highly optimized filters and amplifiers tuned to just one specific frequency (the IF). The RF input is the signal received from the antenna, not the internal frequency being adjusted to cause the conversion. The beat frequency oscillator (BFO) is a different oscillator used later in the detector stage to make CW and SSB signals audible, not the main mixer.