The IRS does not need details of transactions that are properly reported on lines 1a and/or 8a of Schedule D (See exceptions 1 and 2 starting on page 3 of 8949 instructions - link below). These are 1099-B transactions for which proceeds and tax basis are reported to the IRS without adjustments, such as wash sales. All other transactions are reported on Form 8949. If each transaction is detailed on the 8949, nothing further needs to be submitted to the IRS. However, summary totals for each type (A,B,C,D,E,F,X as shown on the 1099-B) may be entered on the 8949. Code M must be entered on lines that report multiple transactions. In this case, the 1099-B or substitute must be submitted to the IRS using Form 8453 - see instructions. It's possible you don't see Form 8453 because you didn't enter code M.
https://www.irs.gov/pub/irs-pdf/f8453.pdf
https://www.irs.gov/pub/irs-pdf/i8949.pdf
