You have to do each year separately. In TurboTax Online enter only your 2021 information.
You cannot use TurboTax Online to file for 2020. TurboTax Online is only for the current tax year, which is now 2021. To file for 2020 you have to use the CD/Download TurboTax software for 2020. You can purchase a download at the following link. There is no free filing for past years.
You might get a better price from a reputable online seller that still has TurboTax for 2020 in stock.
The CD/Download TurboTax software can only be used on a Windows or Mac computer, not on a mobile device or a Chromebook.
Tax returns for past years cannot be e-filed. You will have to print your 2020 tax return, sign it, and file it by mail.
The TurboTax online editions are only for tax year 2021. You Cannot combine tax year 2020 and 2021 on a tax year 2021 tax return. Each tax year is completed and filed separately.
To complete and file a 2020 tax return using TurboTax you would need to purchase, download and install on a personal computer one of the 2020 desktop editions from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/
A 2020 tax return can only be printed and mailed, it cannot be e-filed.