Fidelity’s brokerage business would be covered by SIPC, which would include its cash management accounts. They also likely sweep cash out to FDIC-insured accounts. More importantly though, Fidelity is large enough that you’re unlikely to need that insurance, and that’s really how I’d prefer to approach this.

Yeah we can look at 2008 and say no institution is safe, but if there’s risk everywhere, I’ve just got to try and minimize that as best I can. Fidelity didn’t give me any sort of scare that year fwiw. Disclosure: I’ve been using Fidelity for basically all of my money for most of my career now, including cash management.