Earlier quoted context omitted.
The fact that Robinhood had to emergency-raise $3billion to be able to put up the collateral with their clearing houses should be enough indication that a lot of retail traders were engaged in these highly volatile stocks. See e.g. https://fortune.com/2021/02/02/robinhood-gamestop-restricted...
Robinhood uses its own, internal, clearinghouse.
"Clearinghouses are SEC-registered organizations that act as the central depository for securities. They keep a record of the stocks owned through a brokerage. Clearing brokerages, like Robinhood Securities, are members of clearinghouses."
"The amount required by clearinghouses to cover the settlement period of some securities rose tremendously this week. How much? To put it in perspective, this week alone, our clearinghouse-mandated deposit requirements related to equities increased ten-fold."
https://blog.robinhood.com/news/2021/1/29/what-happened-this...