I think it's becoming increasingly clear what happened. As whitepoplar noted here ( https://news.ycombinator.com/item?id=18691477 ) other similar services are FDIC insured because the funds are swept into FDIC bank accounts behind the scenes. But FDIC insured bank accounts pay very, very little interest, and Robinhood wanted to offer a high interest rate. Their innovation was, instead of sweeping the funds into FDIC…
When Robinhood calls a product a "checking" account that fundamental concept of immediate access to your money is implied to consumers but looking at their site there are all kinds of restrictions around dollar amount and frequency of withdrawals.