Yes, you get React source code, you read it until you understand it => You master React! No, but seriously, what do you mean master React? React is a philosophy, you learn the React framework and become profficient in the React ecosystem.
And now you can call yourself React aware, but not a master, I do not think there are react masters except the persons that actually develop/limit their domain to the React framework itself, so that goes only for the authors & contributors.
Hope this answers a bit!