Getting back to your original question, only U.S. Treasury securities are backed by the full faith and credit of the government and are thus considered as close to "riskless" as a is possible. You can buy short duration T-bills at no cost from most brokerages or pay a fee for the convenience of a Treasury money market fund but make sure it is 100% Treasuries and doesn't include other government securities if absolutely safety is your first concern.
Regular money market accounts like Vanguard's Prime MM or Schwab's sweet funds are primarily composed of commercial paper (short-term corporate debt). Read the fine print. They're not FDIC insured and could lose value ("break the buck") as indeed came very close to happening during the 2008 market crash.
Bank and Credit Union MM accounts and CDs are typically FDIC (or NCUA) insured up to 250K. As long as you stay below that limit safety should be comparable to Treasuries.
Good recent thread on this topic on Bogleheads BTW:
https://www.bogleheads.org/forum/viewtopic.php?t=299856
It's a worthwhile question to be asking in these tumultuous times. I have a lot of my cash position in short-term Treasury funds but decided to keep 6 months worth of living expenses in CDs and MM funds at my local credit union where I can walk in and get physical cash if need be. If things get any scarier with the markets I may look into the old mattress stashing technique.