A backdoor Roth is nothing more than a traditional IRA contribution (or contributions) followed by a Roth conversion. Each of these transactions is entered separately into TurboTax.
A traditional IRA contribution for 2020 is reportable on your 2020 tax return. If it's deductible it will appear on your 2020 Schedule 1 line 19. If it's non-deductible it will appear on Part I of your 2020 Form 8606. Nothing else regarding these transactions will appear on your 2020 tax return.
A traditional IRA contribution for 2021 is reportable on your 2021 tax return. If it's deductible it will appear on your 2021 Schedule 1 line 19. If it's non-deductible it will appear on Part I of your 2021 Form 8606 along with the amount carried forward from line 14 of your 2020 Form 8606.
A Roth conversion performed in 2021 will be reported on your 2021 tax return. The taxable amount of the conversion will be calculated on Part I of 2021 Form 8606 (or using Worksheet 1-1 from IRS Pub 590-B) and the conversion itself will be reported on Part II of the 2021 Form 8606.
