Schwab has an easy-to-understand link on asset protections with the SIPC, additional insurance and the FDIC. Details are similar at other major brokerages, e.g. the Fidelity links.
There are lots of concerns about asset protections after 3 recent bank failures - Silvergate Bank, SVB Bank (#2 biggest failure in the US history), Signature Bank (#3 biggest failure in the US history). Treasury Secretary Janet YELLEN further muddied the current rescue situation by saying that in future, excess deposits may be protected at systemically-important-banks (SIBs) upon presidential determination, but not at smaller community banks. Ironically, deposits at the 2 failed banks under government operations now seem to have have unlimited deposit insurance.