The 737 is that simple, once you select speed on the approach. Autothrottle goes to arm mode, you move the thrust levers manually but if you press TOGA you get go-around thrust.
However the FCTM says in numerous places to disengage the A/T when the Autopilot is disengaged. So Boeing might well have designed the world's greatest autothrottle but they're not letting you use it during manual flying.