Bones are often seen in manga and other comic books in the shape of a stick with bulges at both ends. The bulging parts are joints. The joints of the knee, elbow, and other moving parts of the human body are all made up of these joints. Various muscles attach to the bulges of these joints, and the joints move as these muscles stretch and contract. It can be said that the body moves because the joints move. All joints in the body move according to the same mechanism.
In addition, the movement of the muscles is performed by order from the brain. The brain issues orders for which muscles to move and how to move them in a split second.
We are able to move because our brains give us orders to do so, and muscles do not move on their own without orders.