That flow isn't really data. Like sure, ok, it's data in the sense that everything is data. But that's not the point. The point of retail flow is that it's not based on other market events but is based on mundane things like regular monthly 401k deposits.

This flow is a source of customers for market makers. Those customers get charged a fee. A % of that fee is rebated back to Robin Hood (or other retail brokers) for being originators of the customers.