How to derive "elementary particle field quanta" from "2d string theory world sheets"

I was hoping to see a mathematical and logical derivation:

How to derive elementary particle field quanta (0d creation or annihilation operators) from 2d string theory world sheets? (or do we require not just the world sheets, but also the background spacetime as an input?

Previously I asked a related question but I did not learn such a derivation from the previous posts.