You only get the "Additional" Child Tax Credit (ACTC) if you did not qualify for the full $2000 regular Child Tax Credit (CTC). For example, a filer, with one child, who's tax liability was $600 (less than $2000) might be eligible for $1400 ACTC. But, if his tax liability was $2000 or more, he would not be eligible for any ACTC and would not fill out form 8812.
The child tax credit (CTC) is limited to your tax liability. The CTC is a non-refundable credit and can only reduce your income tax to 0, It can not help you beyond eliminating your tax liability. But, if you have more than $2500 of earned income, some or all of it is usually given back to you thru the "Additional Child tax credit". That is, part of the CTC may be on line 28 of form 1040 (2021- 2024) instead of line 19. The ACTC is calculated on form 8812 and is basically 15% of your earned income over $2500. The ACTC is a maximum of $1700 (2024) ($1600 for 2023, $1500 for 2022) per child, not $2000.
