how to move arms with motor6d when arms and torso are being controlled by an animation?

I know that motor6d works along with animations, I am currently making a gun that shows the players arms when fully zoomed in however when the player looks up the arms don't follow as I expected, I was planning to use motor6d but I have no idea how it works. The animation restricts the arms and torso not sure if that will have any effect on how the motor6d works. Currently, to get it to work I'm trying to rotate the arms up while still in the animation 90 degrees.

game:GetService("RunService").RenderStepped:connect(function()
    character.LeftUpperArm.LeftShoulder.Transform = character.LeftUpperArm.LeftShoulder.Transform *  CFrame.Angles(math.rad(90),0,0)
end)