Why does it only give me a section to enter the advance child tax credit payments [half $3300], and then never populate the remaining that we are entitled to?

Did you enter a qualifying dependent in the MY INFO section yet ?   Or maybe your income is too high ?  

https://www.yahoo.com/now/income-limit-2021-child-tax-155827039.html

There is no place on the IRS forms or in TurboTax (TT) to "populate the remaining (child tax credit) that you are entitled to".

Instead, you get the full Child tax credit ($3000 per child, $3600 for children under 6), but the amount you got, in advance is subtracted, on Schedule 8812, before the net amount is entered on lines 19 and 28 of form 1040.

Furthermore, it is too early to expect the new 2021 TT program to be fully functional yet, with all the changes (especially the child tax credit).