The mod finds the first silo on the planet that's capable of carrying items to the orbit, grabs it selected rocket parts recipe (in case there are modded ones), it productivity and the number of parts per launch. The maximum weight capacity of rockets is a constant as far as I'm aware. It's not the best implementation, it's actually pretty bad, but it works.
That's enough to calculate how many rocket parts it takes to transfer any quantity of items. In vanilla it takes 50 rocket parts to build a rocket, and a single rocket can transfer up to 1000kg worth of stuff, so it's 1 rocket part per 20kg (adjusted for productivity).
It doesn't interact with silos for anything else. Materials for the parts are taken directly from the Storage, in the Storage GUI while in the orbit you can see how many rocket parts are available on the planet below.
