A self employed tax payer pays income tax at the marginal rate plus 15.3% self employment tax.
You pay Self Employment tax (Scheduled SE) on a Net Profit of $400 or more on Schedule C. You pay 15.3% SE tax on 92.35% of your Net Profit (If it is greater than $400). The 15.3% self employed SE Tax is to pay both the employer part and employee part of Social Security and Medicare. So you get social security credit for it when you retire.
The SE tax will be included in your tax due or reduce your refund. It is on the 1040 Schedule 2 line 4 which goes to 1040 line 23. The SE tax is in addition to your regular income tax on the net profit. You do get to take off the 50% ER portion of the SE tax as an adjustment on 1040 Schedule 1 line 15 which flows to 1040 line 10.
The SE tax includes what you already paid in from your W2s so your schedule SE tax will only be the difference up to the max amount of $8,853.60 or social security. The max income for social security for 2021 is $142,800 between W2 wages and the schedule C Net Profit.