How can I get access to the backup/supporting information that went into preparing my tax return?

Only you would have that.  Turbo Tax doesn't have copies of your W2 or any 1099.  Did you import your income or manually enter it?

To access your current or prior year online tax returns sign onto the TurboTax website with the userID you used to create the account - https://myturbotax.intuit.com/

Scroll down to the bottom of the screen and on the section Your tax returns & documents click on Show.  Click on the Year and Click on Download/print return (PDF)

To get a PDF with all the worksheets you will need to wait until December when the ONLINE program comes back online ... at that time download the PDF and the worksheets will be included.  They are not available right now while the 2019 returns are being archived.