"you cannot "attach" 1099-MISC to Schedule C (and thus Schedule 1)"

Because that is not the Task at hand. That is not what is supposed to be happening.

The 1099-NEC is only issued by customers who have paid the taxpayer's business $600 or more, from their business, which validates their business expenditure.

The taxpayer client is supposed to report everything they made in their business. Not only amounts on the 1099-NEC. You don't need to "attach" anything. You need to enter everything. In Gross.

If there are multiple entities, you enter each business' Gross. The 1099-NEC or Misc is informational and not the Amount you enter. You enter the amount based on the operational reality. I already explained that part.

TurboTax is built around prompts, so that the DIY can follow along. Enter everything that got reported about you by your clients to the IRS (all the 1099-NEC info) and then also enter the rest of the info you think the IRS will not find out about your business, in other words. That's how you get them to remember they are supposed to report everything and not hide, for instance, cash payments. Or, as we have discussed in other topics, Barter or Chickens as payment for services.