This is the document that makes payment-for-order-flow visible. It lists the top venues by order type and states net payment per hundred shares, so the economics of a zero-commission broker are public if you look.
Customers may also request order-specific routing detail for their own trades going back six months, which is the practical way to see where a particular order actually went.
Example: a broker's report shows 68% of market orders routed to one wholesaler at net payment of $0.0021 per share received. On 3 billion shares a quarter that is roughly $6.3 million — the revenue behind the commission-free account, disclosed in a table almost nobody opens.
Related: payment-for-order-flow, best-execution, wholesaler, order-routing