The point is that the derivative is the derivative of a function, not a substitution of a particular set of values. The “values” are being back propagated from the “downstream” layers. Do you know what the Chain Rule is and how it works? Note that Prof Ng does not really cover the derivation of back prop because these courses are designed not to require a knowledge of calculus. Here’s a thread that covers the basic derivation of back prop for a feed forward net without dropout. Maybe based on that, it will make more sense if you contemplate how to incorporate dropout into that picture.