The Recovery Rebate Credit, as you outline, is based on your 2020 tax return. The payments in stimulus round one and/or two were "Advances" on the Recovery Rebate Credit. The payments were simply based on those prior returns because they were available. The 2020 tax return is the baseline used to reconcile the credit.
So if, based on your 2020 tax return (2020 AGI), you are eligible for $2400 for round one and $1200 for round two ($3600 total) and you received $2400 in round one, but zero in round two, then your Recovery Rebate Credit on line 30, should be $1200.
Here is a link with more information on the Recovery Rebate Credit.
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