Do you have to file your 2020 taxes in order to receive the additional tax credit?

Which credit? You have to file a 2020 return if you didn't get the first 2 stimulus payments or qualify for more.  The new 3rd stimulus is coming separately.  The new Child Tax Credit increase is for 2021.  

what credit? there are literally hundreds of credits.  generally, to get it, you have to file

If you did not receive the 1st or the 2nd stimulus payment you will have to file a 2020 tax return to receive the Recovery Rebate Credit.

If you are referring to the third stimulus payment, that is controlled by the IRS.  Go to this IRS get my payment website - https://www.irs.gov/coronavirus/get-my-payment

If you are referring to the increased Child Tax Credit that is effective on a 2021 tax return to be filed in 2022.  However, to be eligible for advance payments on the Child Tax Credit for 2021 a 2020 tax return would have to be filed that includes the children as dependents.  See this IRS website for information - https://www.irs.gov/credits-deductions/advance-child-tax-credit-payments-in-2021