First, make sure you save all your tax returns as PDF files. They are universal of course.
Then, each tax return is contained in a tax data file with a filename like "John smith 2018.tax2018". The extension may be hidden by your system settings but it is there and important.
Your 2018 data file can be opened by either a Mac or Windows copy of Turbotax for 2018, but that means that if you really want to open a past year file on the Mac, you have to install a copy of the prior program. (If you are a Turbotax Advantage subscriber, you are given downloads of both Mac and PC versions in your account, and if you bought the program on CD, the CDs have both Mac and Windows version on them. Otherwise, you may have to buy the prior version or persuade customer support to give you a free copy.)
Turbotax versions 2016 and older are no longer sold or supported, so you won't be able to open those data files at all on a Mac, so make sure you have the PDFs. You can make a PDF of "all forms and worksheets."
If you only install Turbotax 2020 on your Mac for the upcoming season, it will read your 2019 tax return in order to import it and get a head start on preparing 2020, but the 2020 program won't open the 2019 program to view the return.
