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.

Robinhood sells to/executes orders through high frequency trading firms like Citadel and (I think) Two Sigma. Because Robinhood isn't connected directly to, say, the NYSE, these HFT firms essentially buy the security and sell it to you for a slightly higher price. TBF, other companies do this too; like, TD Ameritrade, Schwab and E*Trade.

No, HFT firms are obliged to meet the NBBO. They improve prices for retail investors because retail investors are generally safer to transact with.