My understanding (which is limited, so someone please jump in if I'm wrong) is that SIPC does not protect against a decline in value of your assets. I'm wonder

My understanding (which is limited, so someone please jump in if I'm wrong) is that SIPC does not protect against a decline in value of your assets. I'm wondering: First, whether Robin Hood is lending out deposits to margin traders. If not, what are they doing with the money? I don't think that they are, as the article implies, making > 3% on US treasuries. Second, if that investment loses money, are those losses passed on to account holders? If not, someone must be insuring that investment. Who?