What happen to the money that your employer withholds from your paycheck?

The taxes are sent to the taxing authorities. For example, your federal income taxes and Social Security taxes are sent to the IRS. State taxes are sent to your state tax department. Other deductions are sent to the appropriate party. Charity deductions are sent to the charity. Insurance deductions are sent to the insurance company (or kept by your employer if they are self-insured). Savings deductions are sent to the savings institution. 401k deductions are sent to the 401k trustee.