Who determines the collateral required?

The NSCC sets the framework, but it is spelled out in Dodd-Frank that they have to do so by law.

There are some other items, but that's the basic idea - full details are here: https://dtcc.com/-/media/Files/Downloads/legal/policy-and-co...

If you want to dive further i recommend this thread :

https://mobile.twitter.com/KralcTrebor/status/13549526861652...

It’s federal law, part of the Dodd-Frank reforms in the wake of the 2008 financial crisis. I recommend this: https://twitter.com/kralctrebor/status/1354952686165225478?s...

I only knows how it works in otc derivatives and then the collateral is equal to the cost of taking an offsetting position, and is calculated inter-day. But I would guess it is something similar on the stock market.

In this case, the NSCC, or the NAtional Securities Clearing Corporation.