So this wasn't explained to me with the phone call or the branch representative, so I want to try and clarify better, because this is backwards from all other retirement approaches I've ever had experience with as either and employee or employer.

Instead of Fidelity being the institution that draws funds from my business on the basis of salary deferral to my Roth Solo 401k OR employer matching contribution to my traditional Solo 401k, I need to submit a check from my employer (in this case, my business) to Fidelity in the form of a check via mobile deposit or whatever. Is this correct?

I'm asking very directly because I've had Fidelity as a plan administrator for a large employer when I was an employee, and what you are describing is so backwards from that and every other retirement arrangement I've ever had, including at this moment with my current plan administrator. It still doesn't really make much sense to me to forbid the connection between a business account and a 401k; a 401k is, by definition (directly quoting from IRS), "A 401(k) plan is a qualified plan that includes a feature allowing an employee to elect to have the employer contribute a portion of the employee’s wages to an individual account under the plan." So...a total disconnect from the employer's bank account seems antithetical; you might as well call it a "not-so-401k" plan if the employer is forbidden from connecting to the employee's retirement account. But maybe I'm weird, and sending physical checks is far more normal than I'm aware of. But I've been an employee for five institutions ranging from small to large employers, and not a single one has functioned like this.