What app do I use to open download??

What did you download?  The program or the pdf file or the .tax2019 file?  If it downloads as a .txt file, try changing the .txt to .pdf or use a different browser to download it.

On a PC, the download should be an .exe file, just double click to install the program.  On a Mac, the download is a .dmg (disk image) file, just double-click to open the disk image, then drag the icon for Turbotax into your Applications folder to install it. 

Added:

If you downloaded your tax data file (ending in .tax2019 or some other year) that will only open on your computer if you have Turbotax for that year installed on your computer.

If you downloaded your tax returns as a PDF file, you would need Adobe Acrobat reader or another free PDF viewer program.  Most PCs and Macs should have PDF viewing built-in.

If you downloaded something with a different file type, then it has the wrong file type.  Try changing the file type or use a different web browser.