i have not looked into robinhood's margin offering, so maybe it's very different, but in my other brokerage accounts, margin works something like this:
i buy $10k of SPY. after the trade closes, the broker says "ok, you own $10k of SPY, and that's good for another $5k of margin." so then i can go buy another $5k of whatever i want. i pay 9% interest or so on that $5k. if SPY drops in value "enough", the broker can on a moment's notice, sell as much of the SPY as they want to cover my debts.
so... they've got a great guarantee they can take it all back.
And what of the case where the value of your SPY position falls below your $5k+interest debt to them?
Edit: derp. A margin call would cover this.
They do receive payments for order flow.
If you don't pay back your margin they'll come after you like any other debt collector. There's nothing special about it.
The actual risk to the broker is minimal -- they just have to make sure to trigger the sale at a point where the value of all of your holdings (not just what you purchased on margin) is still greater than what you owe them.
When they lend you money, they know exactly how much of that will go back to their hands soon (and they have a ton of cash flow to cover).
Robinhood is effectively lending money with little guarantee to take anything back and no source of income.