You are correct in that this year the Advanced Child Tax Credit you already received is the non-refundable portion that you are referring to in your question. The Refundable portion of the Child Tax Credit is shown on Line 28 of your tax return. This line should have an amount equal to $3,300. Using your numbers you should receive a refund of at least $2,100. Line 35a tells you the amount of your refund.
Basically, this is how it has always worked, except this year you received the non-refundable portion in advance through monthly payments.
