Pokedex entries:
>Necrozma(Dusk Mane): This is its form while it is devouring the light of Solgaleo. It pounces on foes and then slashes them with the claws on its four limbs and back.
>Necrozma(Dusk Mane): This is Necrozma's form while it's absorbing the power of Solgaleo, making it extremely ferocious and impossible to control.
>Necrozma(Dawn Wings): Lunala no longer has a will of its own. Now under the control of Necrozma, it continuously expels all of its energy.
>Necrozma(Dawn Wings): This is its form while it's devouring the light of Lunala. It grasps foes in its giant claws and rips them apart with brute force.
So it sounds like Dusk Mane neither is in control--the act of absorbing the energy causes Solgaleo to enter a mindless rage. For Dawn Wings, Necrozma is unambiguously in control.
The opening FMV seems to portray the fusion as turning Nebby into light and then absorbing it, but the actual in-game cutscene skips that part so I don't know if it's literally absorbing them and then reshaping into the fused forms, or if it just latches onto their physical bodies like a parasitic armor.
