Except I said at the very beginning, including with my own analogy, that the nutrients come from elsewhere. I never said anything about it being an isolated system, because that would clearly be a) impossible just as a general rule but also b) against the concept of parasitism.. Clearly you have never read anything about parasites because "host" is the other half of the parasitic relationship. If I was a "dumbass Republican" I wouldn't be calling it a fetus, nor would I point out its parasitic nature, now would I? In this context it's purely scientific nomenclature, not political, for me to refer to parasite and host.

And do the hosts build the parasitic organism?

No. They supply the nutrients for the organism to build itself. It's not magic, it's biology.