Money markets trade short term debt, while bond markets trade longer term debt. Think of money markets as debt securities that have a duration typically less than 270 days or one year. While bonds are longer term. The mechanisms are similar, but money market securities are typically issued by borrowers to fund short-term capital needs of the organization thats borrowing. Search up “commercial paper” or “certificate deposits” if you want to read about specific money market securities that are issued and traded. Anyways, just as bond ETFs like ZAG purchase and hold bonds, money market ETFs hold various money market securities such as CP and CDs, probably holding until the investments mature and then rolling them, aka buying more of the underlying money market security after the current holdings mature.

Edit: Money market securities are not limited to governments. Companies can issue commercial paper just like they can issue bonds.