This is ONLY if you enable margin trading via Robinhood Gold - they increased their margin requirements recently on GME and this will not happen to you if you are trading on cash.
All accounts are "Robinhood Instant" by default. You need to permanently opt-out of Robinhood Instant (or wait for deposits/trades to settle) to opt-out.
Yes, but "Robinhood Instant" is limited to a subset of the funds you are depositing. It is set up as a margin account to allow them to trade on your behalf before the money hits your Robinhood account. But the money will be hitting, so you will have cash to cover the trade. It's not the same as actual margin trading (where, for example, in this case, the user did not have cash to cover and so his position was liquidated by the broker.)
All accounts are "Robinhood Instant" by default. You need to permanently opt-out of Robinhood Instant (or wait for deposits/trades to settle) to opt-out.
Edit: to add a link. This info is fairly well buried: https://robinhood.com/us/en/support/articles/robinhood-accou...