Where can I find the details of the amount in TT on 1040 Line 2a for 2018 return? The amount exceeds the amount I entered for tax-free Interest Income from 1099Int.

Interest can come from any 1099 you imported.  Or from a K-1 you entered.  Oh, also from a 1099Div box 11

Tax-exempt interest can come from many different sources.  If you are using desktop software, you can use Forms Mode to see the details of the entries on your tax return.  Find your 1040 in the forms list and click on the amount on line 2a.  You will see a small magnifying glass and a link to ''go to supporting form''.   Double-click that icon to see the referenced entry.