I have no affiliation with Simple, but the relationship seems pretty straight forward. "Where" your money is becomes an abstraction. You hold an account with Simple, and are issued a Visa card with which you can access the funds in your account. Everything beyond that is transparent to you.
The accounts would technically be "in your name", since the funds are yours, but you never have to interface with the other bank, so it becomes a question of: do you want to know the technical specifics, or stick to what's pragmatic? Pragmatically, your account would be with Simple. Your money is in an FDIC insured depository bank, so you don't face any additional risk because of the "partner" distribution.