To file an original 2020 return, you would need to purchase TurboTax for 2020 as a cd or download directly from TurboTax or from an authorized 3rd party seller like Amazon (because TurboTax 2020 online is closed).
https://www.irs.gov/businesses/small-businesses-self-employed/deceased-taxpayers-probate-filing-esta...
To file the final 2021 return, you could use TurboTax on a CD or download, or use TurboTax online. If you also have to file a return for yourself, it will usually be more cost effective to use the CD or download version, since each online tax return needs a separate account.
To file an amended 2019 return, you could use TurboTax for 2019, which at this point you can only buy direct from TurboTax.
https://turbotax.intuit.com/personal-taxes/past-years-products/
However, you would first need to exactly recreate the persons original return, mark it as “filed” by printing it (but not mailing), then save, exit, and restart the program to start amending. To create the original return, you would need a copy of their 2019 return and their tax information. Since this can get complicated, you might benefit from professional help.
