If your 2020 tax return showed a refund after applying all of the estimated tax payments that your made for 2020, you could have specified on your 2020 tax return that some or all of the refund to be applied to your 2021 estimated taxes, in which case the amount applied is treated as a Q1 estimated tax payment. If you did not choose on your 2020 tax return to apply any of the refund to your 2021 estimated taxes, the IRS pays the refund to you and none of it is applied to your 2021 taxes.
