Cp2501 received from IRS re cost basis of stock not matching 1099B reported by 3rd party. it appears I submitted all 1099B’s. Why weren’t included in my return?

It depends. If you used TurboTax Online last year, you should have been prompted to include Form 1099-B, with instructions to upload the form. The PDF should have been included in your e-filed return. However, TurboTax for Desktop doesn't support 1099-B upload.

According to this TurboTax article:

If you choose not to upload a PDF of your 1099-B in TurboTax Online, or if you’re using TurboTax Desktop, TurboTax will generate Form 8453. You can still e-file your return, but you'll need to mail a paper copy of your 1099-B to the IRS along with Form 8453, which you can print from your online account or from the desktop software.

Mail these completed forms to the IRS at the address below by the tax-filing deadline:

Internal Revenue Service

Attn: Shipping and Receiving, 0254

Receipt and Control Branch

Austin, TX 73344-0254