How does the Optimo in Dynamo define the optimal results? Thanks!

Hi guys, I’m learning the Optimo of the Dynamo. I’m learning the example case of “Minimum Point on Surface” (here is the web: I guess I can understand most of it, but I can’t figure out where and how it defines what are the “optimal results”. I mean the goal is to find the minimum “z” value on the surface, but I can’t find which part or parts of the node programming are for the judgment that whether the “z” value is the minimum one. I have a kind of feeling that it should be in the “NSGA-II function” part, but when I open it, still can’t find which “nodes” are to determine if the “z” is the minimum or smaller one.
Sorry for my English, I mean there should be some programming like this: " if Z2 < Z1, then "? right?

This is the “NSGA-II FUNCTION”, I don’t find anything like " if z2 < z1, then updating the results" programming. Thanks so much for your help in advance!