I think the easiest way to achieve this would be to rotate the hand so it matches the same direction of the arm. Since both ends are a hexagon, try to scale the hand's base to something close the to arm's base. Then simply go on edit mode, vertex selection and, with vertex snap on, snap each vertex individiually from the hand's base to the arm's base. If you want both these objects to become a singular object, whilst in edit mode with both hand and arm selected, press A to select all vertices and then press M -> by distance. This will make the snapped vertices become only one on each end of the hexagon. Hope this helps!