Distribution of the second round of stimulus payments to individuals began early in 2021. The payment amounts were based on 2019 AGI (obviously the IRS does not yet know 2020 AGI because 2020 returns have not been filed). When you file your 2020 return, you may be able to claim an additional amount in the form of the recovery rebate credit, based on your 2020 AGI.
This is generally adjusted gross income increased by certain items such as tax-free foreign earned income. MAGI usually is used to determine phaseouts of certain deductions and credits.