They don't have to. These are all executive branch departments/bureaus. Trump can do what he pleases without Congressional oversight. They are basically departments overseen by cabinet members.
Obviously they have to on some things: "The reorganization plan faces tough odds in Congress, where even aside from the dispute over work requirements, any reorganization faces opposition from congressional committees that could lose power if their jurisdictions change."
How many departments were established by law? Those would need concurrence of the Congress. But, in general, you are correct about the power of the Executive to organize itself.