From my understanding, zero-commission trades are possible because stock brokers place orders for stocks on third-party platforms instead of placing orders on stock exchanges. The third-party platforms then reward the stock brokers for using their platform. As a result, trades are increasingly moving away from stock exchanges into these third-party platforms. Given that the publicly available bid, ask, and last trade quotes come from the stock exchange, doesn't that mean that these publicly available stock exchange quotes are increasingly unreliable because they increasingly fail to capture the transactions that happen outside of the stock exchange?
For example, if I am looking at the "last trade" price of a stock, this "last trade" price is the price of the last trade on the stock exchange. It probably not the real "last trade" price because there might have been some trades that have happened on third-party platforms outside the stock exchange. So my concern is: to what extent should I trust the publicly available bid, ask, and last trade prices if they aren't necessarily the real prices? Do the prices on third-party platforms have to accurately track the publicly available prices?