If I create an LLC or any other business at a later date that is unrelated to my sole proprietor business, how does that affect my solo 401k?
The LLC or other business should not have any full-time employees, even if the LLC or other business is not the adopting business of the plan. If you own any business(es) with full-time employees, you are no longer eligible for the Solo 401k. A full-time employee is considered to be one who works over 1,000 hours a year and gets a W-2.
If your sole proprietorship is the adopting business of the plan and the LLC or other business does not have any full-time employees (besides yourself and potentially, your spouse), you can continue to use and contribute to your Solo 401k from the income generated by your sole proprietorship.
For further questions, please consult your CPA or tax advisor.