No. The Apple supplied Developers tools needed to create the new 2020 software do not support 10.13 because Apple chooses not to. That is a Apple decision.
See my answer above.
TurboTax requires OSX 10.14 (Catalina) or later. High Sierra is 10.13.
Of course that is for the CD/Download desktop version installed on your Mac. Online versions will work with ANY comparable browser and the OS is not important.
Where is the file kept if I do it online? NOT big on having it in the "Cloud".
On the TurboTax server, but you can download a copy.
Online also does not have all the same features - there is no "forms mode" to view the forms and each account is only good for one tax return.