How are the price signals of stocks in the stock market centralized?
Think of the stock certificate as a physical object being traded around. It may be traded on more than one market. The centralisation you speak of depends on the source for the quote. The quote, generally (some exceptions), is a last trade price (a delayed last trade price). It is not global. I have trading software that allows me to see quotes with less delays, I can choose to see it only on certain markets or last trade from any market. I see different signals than you do. 
The most direct answer to the question in the title is "they're not". There is no central, global signal. What you are likely referring to is just the (delayed) last trade price of the security on the markets that the quote service pulls from.