Biquad cascading frequency response
The application of a filter to a signal in the time domain is defined as convolution.  Convolution in the time domain is mathematically equivalent to multiplication in the frequency domain. So if we filter a signal, and then filter the output of the first filter, this is equivalent to multiplying the signal by both filters in the frequency domain. Therefor, this is equivalent to filtering with a third single filter whose frequency response is equal to the product of the first two filters’ frequency responses. Because frequency responses are complex numbers, the resulting magnitude is equal to the product of the two input magnitudes and the resulting phase is the sum of the input phases.  This can be easily extrapolated for more than two filters as well.