How to play animations Over top of varying character limb positions?

Hello, I didn’t know how to properly write a title for this since it is complicated but I was wondering if anyone knew how I would play animations that move the characters arms while the arms are already being moved to follow the mouse up and down. I’ve read of people using the motor6d to make the arms follow the mouses y movement but I don’t know how to play animations over top of that, would the animation override the current cframe of the arm and reset the arms position?. I’m trying to go for the look of FPS games like phantom forces, although I’m not sure the method that game uses for this, maybe they make “fake” arms and do it that way but I don’t know enough about animations and cframes to have an answer currently. Any help pushing me in the right direction would be much appreciated