It's the same as a nondeductible contribution + a conversion in 2021.

Example here: https://www.merriman.com/wealth-enhancement/how-to-report-a-backdoor-roth-ira-contribution-on-your-t...

As long as the person has no other deducted contributions or earnings in any Traditional IRA, this will be nontaxable Conversion. Otherwise, this nondeductible amount is part of the % that is not taxable, but the Total basis and Total account(s) balance(s) across all Traditional IRA accounts is used for how much will be taxable upon conversion. See this article:

https://www.irs.gov/instructions/i8606