What is the major source of government waste and fraud in the US taxpayer system?

According to the clip, foreign nationals using fake Social Security numbers and identities are stealing billions in taxpayer benefits. This organized fraud is occurring across multiple agencies including the FDA, Medicare, and Medicaid. The commentator suggests that by addressing this massive fraud in tax and entitlement systems, the government could potentially save over a trillion dollars over a 10-year budget window. Meanwhile, the Biden administration has focused on hiring 87,000 IRS agents rather than tackling these systemic issues.