"Backdoor" relies on Basis. That means it's the same as Roth IRA money (no tax benefit for the contribution), but not put directly into a Roth IRA account. That means it is not deductible and it is tracked.

When you convert Basis to Roth, since it is already post-tax, there is no taxable event. However, the "backdoor" process only works when this is only Basis. Let's review:

They can't have any non-Basis funds in any Trad IRA, SEP IRA or SIMPLE IRA. These account types are aggregated for purposes of the conversion. If they have any pre-taxed, rollover or contribution or earnings, in any of these account types, their Backdoor is now a commingled conversion subject to pro-rata taxation.

And once there are commingled funds, every conversion is not only partially taxable and partially not, but the partially Not Taxable amount is tracked, as it will reduce Basis. Then, the next conversion needs to take this updated Basis figure into consideration.