A snippet from one of the IRS letters (bold is mine):
How the IRS determined your payment amounts Monthly payment amounts were initially based on information from an income tax return you filed or information you entered in the IRS non-filer sign-up tool in 2020 or 2021. Your monthly payment amount or how or where the IRS paid your payment may have changed based on information you provided the IRS through your 2020 income tax return if the IRS processed it after June, the Child Tax Credit Update Portal, or the dedicated IRS Child Tax Credit phone line. Review each monthly payment, including any changes, at IRS.gov/ctcportal, and click “Manage Advance Payments.” If you did not receive one or more payments, contact the IRS at 800-908-4184 before filing your return.
We all know how well the IRS is at answering their phones!