If you're willing to read a lot, this is a decent overview:

https://www.stockbrokers.com/guides/order-execution

Also here's the disclosure from IBKR, which is probably the polar opposite from robin hood in terms of favorable order execution

https://gdcdyn.interactivebrokers.com/Universal/servlet/Regi...

I know the first link said fidelity is better, but that's fidelity vs ibkr-lite, not ibkr-pro.