How does a mega backdoor Roth differ from a regular backdoor Roth?

They exploit different rules. A regular backdoor Roth moves money into a Roth IRA: you make a nondeductible Traditional IRA contribution (capped at the standard IRA limit) and convert it, sidestepping the Roth income limits. A mega backdoor Roth happens inside a 401(k): you make after-tax (non-Roth) contributions above the normal deferral limit, up to the overall IRS defined-contribution ceiling, then convert those dollars to a Roth 401(k) or roll them to a Roth IRA. The mega version can move far more money, often tens of thousands, but only works if your employer's plan explicitly allows after-tax contributions plus in-plan conversions or in-service withdrawals. Confirm plan features with HR; the IRS sets the overall limit annually.