How do they know I paid my estimated quarterly taxes?

it won't know since Turbotax and virtually all other tax apps don't assume the payments are made. you'll have to enter them on the tax payments worksheet.  in step-by-step mode Turbotax should ask you if you made these payments. then it will take you through entering each payment amount and the date.  if you pay state estimates you also have to enter those. 

You have to manually enter them.  You can type estimates paid in the search box at the top of your return and click Find and it will give you a link to Jump To it.

OR

To enter Federal or State Estimated Taxes Paid, including a state estimated payment made in January for the prior year, go to

Federal on left or at top.  Personal (Home & Business)

Deductions and Credits at top

Then scroll way down to Estimates and Other Taxes Paid

Estimated Taxes - click the start button

For 2020 Federal Estimated payments should be on 1040 line 26.