For 2021, 2022 and 2023, you will need to purchase Turbotax Desktop version as a download to install on your own Mac or PC. The link is here.
https://turbotax.intuit.com/personal-taxes/past-years-products/
2024 is not "back taxes", that is the current year. You can buy the download for the 2024 version or you can use Turbotax online.
2020 is no longer supported by Turbotax. You would need to see a professional or download the forms and instructions from the IRS web site and prepare your taxes manually.