Hello there, yes, you do owe self employment tax even if you are fully employed. If your side hustle pays cash or you receive a 1099nec or 1099misc you must report the income on Schedule C.
Basic filing requirement for Self-employed taxpayers:
You have to file an income tax return if your net earnings from self-employment were $400 or more. If your net earnings from self-employment were less than $400, you still have to file an income tax return if you meet any other filing requirement listed in the Form 1040 and 1040-SR instructionsPDF.
If you Self Employed income is below $400 and you are required to file a tax return because of other income earned, then you are required to report the Self-Employed income as well because once we are required to file we must report ALL worldwide income earned.
You can see if you are required to file by referring to chart A, B, or C in the 1040pdf below.
https://www.irs.gov/pub/irs-pdf/i1040gi.pdf
Additional Resources for Self-employed
https://www.irs.gov/businesses/small-businesses-self-employed/self-employed-individuals-tax-center