I am surprised that this is not more commonly known. This should not be confused with a Backdoor Roth IRA. Per Fidelity, a Mega Backdoor Roth IRA requires 2 steps:

(1) making after-tax contributions to your 401(k) or other workplace retirement plan, and

(2) then doing a conversion either to a Roth IRA or Roth 401(k)"

What is a mega backdoor Roth? | IRA conversion | Fidelity

It is also possible to do the rollover with earnings (see below).

How To Enter 2022 Mega Backdoor Roth in TurboTax (Updated) (thefinancebuff.com)