Don’t get confused by the venturi; it’s only there to cause a low pressure region, so that the fuel can be drawn from the jet/jets into the throat of the carburettor.
The throttle butterfly plate is situated after the venturi and the MAP is measured after the throttle plate; for a non-supercharged engine at near constant rpm, the position of the throttle is what determines the MAP.
It might help to think of the engine as a fuel/air pump (that’s what it is), powered by the fuel air mixture itself.
The engine is trying to pump / suck all the air/fuel from the inlet manifold, but the atmosphere tries to replace it. If the throttle is closed, the engine wins and the MAP reduces. If the throttle is wide open, the atmosphere catches up and increases the MAP.
Increased power doesn't result in higher MAP, it's more correct that a higher MAP gives more power.