Retirement plans fall under the jurisdiction of both the Department of Labor (DOL) and the Department of the Treasury/Internal Revenue Service.
Further, plans are generally governed under the rules and regulations of the Employee Retirement Income Security Act of 1974, also known as ERISA.
One participant plans like the Solo 401k are not generally subject to ERISA if the participant is the owner of the business because there are no common law employees.
Solo 401k plans are Qualified Retirement Plans in the eyes of the IRS but are not covered under Title 1 of ERISA (which typically deals with employees rights, eligibility, funding, reporting and fiduciary rules).
Title 1 only covers employees other than you.
ERISA was originally created to ensure no shenanigans happened in retirement plans where the employees got cheated or mistreated.
Since you are the only employee in your retirement plan, Title 1 doesn't apply to any Solo 401k plan
However, you still are afforded the general protections of an IRS approved Qualified Retirement Plan. Plans not subject to ERISA are subject to the parallel provisions of the IRC such as the prohibited transaction rules, non alienation of benefits, QDROs, etc.