A good overview of the flow of volume through these various venues can be found here. Bottom line - there are more than two exchanges. https://www.cboe.com/us/

A good overview of the flow of volume through these various venues can be found here. Bottom line - there are more than two exchanges. https://www.cboe.com/us/equities/market_share/ https://www.cboe.com/us/equities/market_share/