If you do not opt out then you get half the CTC before the end of 2021 and the other half on your 2021 tax return. The amount of the credit is the same; it is just a matter of WHEN you receive it.
Yes that is correct. Half of the child tax credit is refundable so you will still have a child tax credit amount on your tax return. Any other refund will be based on withholdings, etc. or earned income credit if you qualify.
Q. So, if I generally receive a refund, I should still receive one next year even if I do not opt out. Is that right?
A. Simple answer: Yes.
But taxes aren't simple. The Child Tax Credit (CTC) increased from $2000 to $3000 ($3600 for children under 6). So if you get half that $3000 in advance; that means you'll get $1500 next year when you file your tax return. So, the simple answer is your refund will be $500 less next year. That assumes you got the full $2000 the previous year. Another change for 2021 is that the CTC is fully refundable. That is, not everybody got the full $2000 in the past. They will get the full $3000 for 2021 (unless their income is very very high), half in advance payments and half a tax filing time.