A Roth IRA is one type of retirement account.
A Traditional IRA is another type of retirement account.
A Solo 401k is yet another type of retirement account. Your Solo 401k comes with a built-in Designated Roth Account. The Designated Roth Account is not a separate, standalone account, but is actually a sub-account of your Solo 401k.
The designated Roth account in your Solo 401k allows you to contribute Roth (after tax) funds and do in-plan conversions where funds in your Solo 401k can be converted to Roth funds (after paying the tax, please consult with your CPA on how this affects you and for proper tax calculations).
A Roth IRA can only be rolled into another Roth IRA.
If you have funds in a Roth IRA, they must remain in a Roth IRA vehicle.
Funds in another non-Roth IRA account (e.g. Traditional IRA, 401k, Roth 401k, 403b, TSP, etc) can all roll into your Solo 401k.