Thank you very much @dmertz for sharing your wisdom with the community.
@dmertz wrote:If you have not reached age 59½, the amounts of your contributions, conversion and earnings in your Roth 401(k) retain the same nature and add to the corresponding amounts in your Roth IRAs.
I found what you said about the Roth 401K contributions being treated the same in the Roth IRA account after the rollover in the section "Determination of 5-Taxable-year Period after a Rollover to a Roth IRA" of https://www.irs.gov/irb/2007-22_IRB#TD-9324
If a nonqualified distribution from a designated Roth account is rolled over into a Roth IRA, the portion of the distribution that constitutes a nontaxable return of investment in the contract is treated as basis in the Roth IRA.
However, I couldn't find that the earnings on the Roth 401K become earnings on the Roth IRA. This logic of treating them the same makes sense, but I want to double-check that I am getting things right.
