Better explanation for the layman: https://startupsventurecapital.com/robinhoods-exceptionally-...

Some detailed links on payment for order flow:

https://www.sec.gov/fast-answers/answerspayordfhtm.html

https://www.sec.gov/fast-answers/answersmktmakerhtm.html

https://www.sec.gov/reportspubs/investor-publications/invest...

https://hub.ipe.com/find-research/white-papers/payment-for-o...