If you "hold a share"at Robinhood, you do NOT own the share. What you own is an IOU for a share.
So when you buy or sell there, they buy it in their dark pool, and sell it on the market. And give/take whatever difference in cash you paid to you without you ever actually owning the share. They may do this without ever actually holding the share though.
If you transfer OUT of their dark pool however, and if they don't actually have a share to be represented by the IOU they sold you, they have to buy one at whatever price they can get in order to deliver it to whichever on the market broker you prices for transfer through- like Fidelity or TDA.
Market Data 
Markets 
