I have qualified dividends (1040 line 3a) and ordinary dividends (1040 line 3b). Amount in 3b is added to income and taxed accordingly. Where is amount in line 3a used?

Yes -- qualified dividends are used in a special tax calculation.

When you enter qualified dividends and/or capital gains on your return, taxes will not be calculated using the tax tables, but using either the Schedule D Tax Worksheet provided in the Instructions for Schedule D, or the Qualified Dividends and Capital Gains Tax Worksheet in the Instructions for Form 1040 and 1040-SR.

Thank you @JohnW152 
I can’t confirm whether the amounts are calculated properly for me because TurboTax online is not showing the worksheet. I already submitted taxes. 
It was mentioned in another post that only TurboTax desktop software allows to download or view the worksheet. That’s a hassle. 

Once you complete your tax return you have the ability to save a .pdf copy for your files.  Any worksheet that was used in your tax return will be part of that file.  You can use this either of the links to create your .pdf copy and print or review it. 

• How do I save my 2020 TurboTax Online return?

• How do I get a copy of my 2020 TurboTax Online return?

There is no cause for alarm, if you entered the qualified dividends correctly when entering your 1099-DIV then TurboTax made the correct calculations for your tax return. This was not something that was new this year.

I know about the pdf copies. But it doesn’t download the worksheets used.