If you don't file the 1099NEC on schedule C the IRS will see the whole 1099NEC as income and send you a bill for regular tax on it and the 15% self employment tax. They won't know about your expenses.
Self Employment tax (Scheduled SE) is automatically generated if a person has $400 or more of net profit from self-employment. 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 (FICA). So you get social security credit for it when you retire.
