Most often, self-employment tax is paid because you've entered self-employment income, either as cash or from a 1099-NEC, 1099-MISC or 1099-K form. If your self-employment income is $400 or more (after deducting expenses), you'll have to pay self-employment tax.
Other less common reasons to pay self-employment tax:
You have clergy income.
Your income as a church employee exceeds $108.28.
You received income from a partnership (Schedule K-1).