I missed an inherited Roth IRA RMD in 2023. I've already filed my 2023 taxes. How do I file a form 5329 and pay the penalty? Do I need to also amend my 2023 taxes?

Inherited Roth IRAs don't actually have RMDs.

It's a technicality.  Technically, inherited Roth IRA's have the same RMD rules as inherited traditional IRAs.  But, you only need to take an RMD from an inherited traditional IRA if the original owner was older than their RMD beginning age when they died.  Because Roth IRAs don't have an RMD requirement for the original owner, they don't have a beginning age.  And since they don't have a beginning age, the beneficiary is never in a situation where the original owner was "older than their RMD beginning age."

And, if you don't believe that technicality, there is a second technicality.  Because the RMD rules for inherited IRAs were created by the SECURE act in 2020, but the IRS did not finalize the rules until 2024, penalties for missed RMDs on inherited IRAs are waived from 2020-2024, and you only need to start taking the RMD in 2025 (except Roth IRAs are exempt for the reasons explained).