Q. I worked for someone I'm not self employed. So how do I file?
A. You file as self employed.
Although it may be news to you that you “have a business” (and most of the other people who get their first 1099); The IRS considers anything on a form 1099-NEC to be self employment income. Enter the 1099 income, at Other Common income / income from a 1099-NEC . TurboTax (TT) will complete Schedule C for you and allow you to deduct any expenses associated with this income. You'll also have to pay self employment tax (social security & Medicare) on any profits greater than $432. These types of payments are frequently done this way. Your "employer" is not the only one.
The IRS also considers undocumented cash income (no W-2 or 1099), for work performed, to be self employment income.
The IRS considers you to to be self-employed if you worked as an independent contractor and were paid by someone who did not withhold any taxes, Social Security or Medicare from your pay. If you qualify to use it, you can switch to a software version called Free File instead of the expensive online Self-Employed software. You have to be able to prepare a schedule C for your business income and you will be paying self-employment tax.
https://ttlc.intuit.com/questions/2903027-how-do-i-report-income-from-self-employment
https://ttlc.intuit.com/community/self-employed/help/what-is-the-self-employment-tax/00/25922
https://ttlc.intuit.com/questions/2902389-why-am-i-paying-self-employment-tax
https://ttlc.intuit.com/questions/1901340-where-do-i-enter-schedule-c
https://ttlc.intuit.com/questions/3398950-what-self-employed-expenses-can-i-deduct
Try Free File:
You qualify if your income was $39,000 or less, or $72,000 or less if active duty military, or if you qualify for Earned Income Credit
https://ttlc.intuit.com/questions/1900583-what-is-turbotax-free-file-program
Or—-Use this IRS site for other ways to file for free
https://apps.irs.gov/app/freeFile/