When you prepare your tax return for 2020 in TurboTax, you will be prompted to enter how much you received for your stimulus and then the program will calculate the amount of stimulus due you based on your income, number of dependents and payments already received. If any additional stimulus is due you, it will be used to reduce your 2020 income tax liability, and the balance will be refunded to you.
You will see the options when you are done entering your tax information and proceed to the review process.
You can file a tax return in 2020 to get your stimulus check, even if you aren't required to otherwise.
I you are due a stimulus payment, it will appear on line 30 of your form 1040. It is called Recovery Rebate Credit.