For example, if India knows about China, and has maps of it, they could be traded to the Arab world, who pass it to Europe.
I just don't understand how A can know B and B can know C, yet A doesn't know C.
It's not that they can't be, but that they weren't. Accurate maps leading up to the period that I study, and continuing to some extent through it, were treated essentially as state secrets -- being able to accurately plan voyages and navigate, in an age where sea-based commerce and expansion were driving growth in Europe, was knowledge that people were reluctant to share with adversaries.