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

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.