AliciaP1's reply cannot possibly be correct given the recent IRS guidance regarding Roth employer contributions. That guidance makes the Roth employer contribution taxable to the employee in the year that the deposit by the employer is made, not some future year, so there is no special tracking of the Roth employer contributions to make them taxable when distributed.
IRS Guidance (see Q&A L-9): https://www.irs.gov/pub/irs-drop/n-24-02.pdf
Interpreting this IRS guidance, for a Roth employer contribution to a solo 401(k) for 2023 deposited in 2024, I would expect that it would be necessary to report the employer contribution as a traditional 401(k) employer contribution included in the self-employed retirement deduction on 2023 Schedule 1 line 16, and then report a 2024 In-plan Roth Rollover by providing to yourself (as both employer and employee) and to the IRS a 2024 Form 1099-R with the employer contribution amount in boxes 1 and 2a and code G in box 7, making it taxable on your 2024 tax return. Consistent with the IRS guidance for Roth employer contributions, this results in the Roth employer contribution being taxable to the self-employed individual on the tax return for the year in which the deposit is made.
For a solo 401(k) providers that have a plan agreement that allows Roth employer contributions, I might imagine that the provider would issue on behalf of the employer the Form 1099-R required to report the In-plan Roth Rollover. Contact your solo 401(k) provider for more information.
