I’ve been staring at this for a couple hours, trying to understand it, finally the stack denotation made sense to me, it is fascinating! But then he completely

I’ve been staring at this for a couple hours, trying to understand it, finally the stack denotation made sense to me, it is fascinating! But then he completely lost me at “This is not a Functor morphism!”, maybe I’ll come back another day to try to make sense of it