The law requires employers to withhold taxes from employee earnings to fund the Social Security and Medicare programs. These are called Federal Insurance Contributions Act (FICA) taxes. Your employer also pays a tax equal to the amount withheld from employee earnings.
The self-employed pay Self-Employed Contributions Act (SECA) taxes on net earnings. SECA taxes also fund Social Security and Medicare. The self-employed pay both the employee and the employer share of SECA. But the law permits them to deduct half of the self-employment tax as a business expense.
Fact Sheet: 2021 Social Security Changes
Retirement Benefits: Maximum Taxable Earnings
If You Are Self-Employed