They don’t just give the market makers data, they match them with customers. That’s payment for order flow. This model is slightly threatened since some people don’t like it and even the SEC chairman has spoken against it (I said “under attack” above and that’s a bit dramatic).

Exactly, PFOF does not involve selling data. The spread available on the stock market reflects the potential toxicity of random orders to market makers; it is risky to market make large volumes of a security to an unknown counterparty (who might know more than the MM). The spread pays for this risk.

It is not as risky to market make securities for retail investors; they probably don't have insider knowledge and are low-volume. So, PFOF routes retail orders to the market makers, where it likely executes at a better price/spread than the public market and also gives the brokerage a tiny commission in the process. And is still regulated by NBBO...