Because it's possible to make irrecoverable mistakes related to retirement savings, I've spent many years of reading the tax code and IRS forms, instructions, Notices, Revenue Rulings, Revenue Procedures, Treasury Decisions and Written Determinations related to retirement savings so that I know where to find the answers needed to avoid making such mistakes with my own retirement savings. All of this information is available on the internet.
In your case, the answers to what happens with rollovers from a Roth 401(k) to a Roth IRA are in IRS Treasury Decision 9324 and in the safe-harbor explanation created by the IRS that the 401(k) plan can use provide the required explanation to the individual prior to making the distribution from the Roth 401(k) account. The safe-harbor explanation is given in IRS Notice 2020-62.
