broadly speaking you are correct... if your income was high in 2018/2019 but is low in 2020, then that should result in more stimulus paid to you via the tax return....
broadly, the IRS takes your 2020 circumstance (income, filing status and dependents under 17) and determines how much stimulus that calculates to, then it subtracts what you have received and the difference is on line 30 of Form 1040 as a credit which increases your refund.
