This newest round of stimulus payments (EIP 3) will be reconciled on the 2021 tax return. However, the prepayment will be based on the most recent information, between 2019 and 2020, that is on the IRS’s systems at the time of payment in late March. This includes your income and your dependent information.
If the IRS has processed your tax year 2020 return by the time your stimulus funds are disbursed, then you will receive stimulus funds based on your 2020 tax information. If the IRS has not processed your tax year 2020 return by the time your stimulus funds are disbursed, then the IRS will use your 2019 tax information to process your stimulus payment.