Tax data can only be transferred from the prior tax year to the current tax year return.

So only the 2019 tax data can be transferred to the 2020 tax return.

No.  You can only transfer from the immediate prior year, do from 2019 into 2020.  You need to have the .tax2019 file on your computer to transfer from.

How to transfer into Windows Desktop

https://ttlc.intuit.com/community/entering-importing/help/transfer-last-year-s-return-to-turbotax-fo...