The new (expanded) Child tax credit ($3000; $3600 for kids under 6) is effective for 2021. There is a provision for advanced monthly payments being made in 2021. The payments will come from the IRS, similar to the stimulus payments.
Getting the expanded child tax credit has nothing to do with filing your 2020 tax return. Whether you already filed, file now or file later, the additional money will not be part of your 2020 tax refund. Nor will the $1400 third stimulus.
There will be monthly child tax credit payments going out beginning in JULY 2021, with some of the CTC still left as a refundable credit on your 2021 tax return next year.
use this IRS portal