The IRS already withheld enough money to pay for this year taxes, how turbo tax import the actual withholdings in to the into the calculations?

You enter your withholding amounts when you enter your tax forms such as your W-2s (box 2) and different kinds of 1099s.   There are many different kinds of 1099s, so the withholding amount, if applicable, is in a different box on each of those forms.    TurboTax will calculate the amount of your tax liability and you provide the amounts that have already been withheld.  The difference is part of your tax refund or tax liability.  

TT can import nothing from the IRS.   you say IRS withheld but are using self-employed. so it's not clear what you're referring to. if you made estimated tax payments for 2019 then they get entered in  Tax payments wks.   if you have withholding forms all the info from those forms gets entered into TT and the withholdings from those forms transfers into your returns.   

How did the IRS already take it?  Was it withholding out of your paychecks or on 1099 income?  Any withholding taken out will show up on your 1040 line 17 and be subtracted from the total tax calculated on all your income.  The withholding is just an estimated amount to cover the actual tax on your income.  So you either get a refund if too much was withheld or you will have a tax due if you didn't send in enough during the year.

Do you have self employment income?  There is no withholding on that.  You usually need to send in extra quarterly estimated payments yourself directly to the IRS to cover the income tax and the self employment tax.  You have to enter any estimated payments you made.  They will show up on 1040 line 18d.