I assume you are referring to what cells I am detecting with myoelectric sensors?
If you look at your fingers: it’s all skin, bone and some pads of fat. Not much muscles there so how do you move your fingers? The motion of all your fingers are controlled by muscles in your forearm. So when you make a tight fist you will notice that your forearm tenses up.
These muscle cells are controlled by electrical signals sent from the brain. When that electrical signal reaches the muscle it “moves” along its length, which can be detected by myoelectric sensors.
Here is a quick video of someone using a myoelectric prosthetic, you can see the end of his residual limb twitching and controlling the hand.