It is the payroll tax you were already paying
An employee sees 7.65% of their wages go to Social Security and Medicare. What they usually do not see is the other 7.65%, paid by the employer on top of their salary.
Work for yourself and you are both parties, so you pay both halves: 15.3%. Nothing new has been invented; a cost that was always there has stopped being hidden.
It is charged before income tax and independently of it. Somebody who owes no income tax at all can still owe self-employment tax.
The 92.35%
The tax is not charged on your whole net profit. It is charged on 92.35% of it.
The reason is symmetry with employment. An employer deducts its half of payroll tax as a business expense, and the employee is never taxed on it. The 7.65% reduction to the base does the same job for somebody who is both.
Half of the resulting self-employment tax is then deducted against income tax, for the same reason. Neither adjustment is optional and neither is a loophole.
A job changes the arithmetic
Social Security tax stops once your earnings reach the annual wage base. Medicare never stops.
Wages and self-employment income share one base, and wages fill it first. If a job has already used it up, the Social Security part of the tax on a side business is zero and only the 2.9% Medicare part remains.
This is why a generic calculator overstates the bill for anyone with a job. Applying 15.3% to side-business profit without asking about wages can be wrong by thousands.
What reduces it
Business deductions do, because they reduce net profit, which is the base. This is why a deduction is worth more to somebody self-employed than the income-tax rate alone suggests: it saves income tax and self-employment tax together.
The qualified business income deduction does not. It comes off taxable income later and never touches the self-employment tax base.
An S-corp election can, by splitting profit into a salary that carries payroll tax and distributions that do not. It brings a separate return, payroll filings and a salary you have to be able to defend, so it is a real decision rather than a free saving.