The "Mega-Backdoor Roth" contribution option moves funds from a Traditional 401(k) -> Roth IRA while the "Backdoor Roth" contribution option moves funds from a IRA -> Roth IRA.
The Plan Administrator is not required to file anything for contributions to the Traditional 401(k). Filings for contributions to an IRA are reported on IRS Form 8606.
When the distribution or rollover occurs which moves the money from the Traditional 401(k) to the Roth IRA, the Plan Administrator will issue a form to the IRS on 1099-R. You should receive a copy of this form, which you will file with your taxes. If the contribution was performed and recorded correctly, the taxable amount on this form should be 0 (or "not calculated"). To see a copy of this form, visit PDF Form. See Instructions.
If you are the Plan Administrator, and the total assets in your 401(k) are worth more than $250,000, IRS Form 5500 needs to be filed. The Plan Sponsor may need to file additional forms.