Assets held in your brokerage account that are not swept to FDIC-insured program banks are protected by SIPC, which protects securities customers up to $250,000 in cash claims, or $500,000 in total inclusive of securities.
Once cash is swept to a program bank, it is no longer within your brokerage account; thus is not protected by SIPC, but is eligible for FDIC protection of up to $2.5 million at a network of program banks, subject to FDIC limitations.