If the beneficiary is not a spouse, the Roth solo 401k funds would get directly rolled over to a “beneficiary Roth IRA” for each non-spouse beneficiary, and they would then be required to empty out their respective beneficiary Roth IRA within 10 years (the 10 year rule which was plumaged by the SECURE ACT) . This means the beneficiary ROTH IRA will need to be fully distributed by December 31st of the tenth year following the solo 401k owner’s year of death. The Roth IRA distributions will be distributed tax free, of course.
Note: Please also note in the event that you die before the 5-taxable-year period of participation with respect to your Roth 401k has passed, the non-spouse beneficiary will need to wait until 5-year period has been met in order to be able to withdraw the funds as a “qualified roth distribution” (note: qualified roth distribution requirement of taking the distributions “after your death” will automatically have been satisfied given your death). See discussion of “What is a qualified distribution from a designated Roth account?” on IRS page HERE.