I’m not certain if I’m completely correct here, but as I understand it - many of these trading platforms will allow you to use the money you deposit into their

I’m not certain if I’m completely correct here, but as I understand it - many of these trading platforms will allow you to use the money you deposit into their system immediately. The actual transaction that fulfils that deposit often takes a day or two to complete. If you deposit $100 and use it to immediately buy $100 of GME, RH and the other platforms effectively borrow money to fulfil that trade. At scale, if they have a large number of new customers making use of that facility simultaneously they may actually exhaust the amount of credit they have access to for the given amount of collateral they’ve fronted to their lender.