Your confusion arises from an understanding that there will be multiple exchanges(A,B,C), that won’t be the case.
When market coupling is implemented, the MCO(Market Coupling Operator) becomes the sole exchange, and current players like IEX, PXIL and HPX get reduced to brokers. An analogy would be, MCO will be NSE and current exchanges like IEX will be brokers like Zerodha, Angel One etc.
So in this scenario, all transactions occur on MCO while the role of IEX will be primarily not of price discovery but just collection of buy/sell bids. This is the popular understanding of MCO implementation, if there is a twist we will have to wait and watch.
This article gives a good introduction to overall business of IEX: