What's the relationship between output impedance and input impedance?

In one scenario I have a voltage buffer with an output impedance, which in some way should relate to the input impedance of a bandpass filter.

In another scenario, I can calculate the output impedance of a low pass filter and the input impedance of a high pass filter, in a band pass filter. But how do they relate to each other?

Should I be performing some be adding/multiplying/etc the output impedance with the input impedance or include the output in the equation for impedance?

It's said that an output impedance of a voltage buffer is fed into the bandpass filter.