Without the letter number, I'm guessing the IRS wants to verify that your wage and withholding information is correct. It's possible one of your employers did not send the IRS your W2, or you reported information different from what the employer reported to the IRS, so the IRS is asking about the discrepancy.

The IRS did not ask for self-employment information so don't send that. However, be sure to keep good records of your self-employment income and expenses in case they are requested in the future.

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you have to pay taxes even if individual jobs don’t have to issue 1099s. Just making under $600 per job (the 1099 threshold is $600 not $400) doesn’t mean you don’t owe taxes on the income. You should amend your past 3 years of tax returns to properly reflect your work, which will likely be reported on a schedule C. More on reddit.com

Do not pay yourself a salary through payroll. As a sole proprietor you will report your income and expenses on schedule c and be taxed (self employment and income) on the net income/profit. If you are not using Quickbooks Online or a similar accounting system you need to start now. Another response here is correct - you will need to have multiple years tax returns to prove income depending on the lender. More on reddit.com

Not sure if it fits with your industry, but look into LLC with s-corp election. More on reddit.com