Well the Second Stimulus payment for 2020 could have come in Jan or Feb 2021. That was still reported on your 2020 return. The 3rd Stimulus payment came out starting in March 2021 and is for 1,400 per person. That is the only one you reconcile on your 2021 return. The IRS should have sent you letter 6475 to tell you how much they sent you. If you are married you should each get letter 6475 for half.
The first EIP --or stimulus check--- was issued in 2020 It was for $1200 for adults and $500 for a dependent under the age of 17. The IRS used information from either your 2019 or 2020 tax return to issue those checks so they could send them out in a hurry.
The 2nd EIP or stimulus was issued in late 2020 or early 2021 and was $600 for adults and $600 for dependents under age 17.
If you did not receive the 1st and/or 2nd stimulus check you can get it by filing a 2020 tax return and using the recovery rebate credit in Federal Review. It will end up on line 30 of a 2020 tax return and will be added to your refund (or reduce your tax due).
The 3rd EIP--or stimulus check-- was sent out in mid-March or later in 2021 and was for $1400 per person--adults and dependents no age limit.
If you did not get the 3rd EIP payment you can get it by filing a 2021 tax return and using the recovery rebate credit in Federal Review. The credit will go on line 30 of your Form 1040 and will add to your refund (or reduce your tax due).