C2W1 where is a_in assigned

In reviewing the code for exercise #2 in c2w1 problem set, I’m having difficulty finding where the values for a_in are assigned. Can someone explain where this happens in the code, please. thank you

Hi @Joshua_Neft

a_in is the input of the function dense(output of the previous layer) so in this image
a_in first is x and after that a_in = a of the output of the previous layer