You stated this part incorrectly: "in Feb 2021 for 2020, then converted it to a ROTH IRA for 2020,"

The Conversion is for 2021 tax reporting. They obviously made the conversion in 2021.

The "backdoor Roth" function is a Conversion made timely enough, and from a person who only has basis in their Traditional IRA (and similar) accounts, that there is no Taxable amount from (prior deducted contributions and earnings).

The $6k example, put in Feb 2021 against 2020 tax filing (form 8606) is then Converted in 2021. It is reported on a 1099-R for 2021, fill in the form 8606 for 2021 tax reporting, and show what was converted was 100% basis.

If they have anything other than Basis, it isn't nontaxable, but partially taxable. It would be pro rated against Basis.

This also assumes this was done trustee-to-trustee (or same trustee) and there is no Withholding to overlook for Gross vs Net.