Go into vertex edit mode and adjust the size and shapes of the digits so that they more closely resemble the appropriate lengths for fingers.

Select the edges of all the fingers and connect so that you now have a new edge in the center of each finger.

Switch to perspective view and along the top of each finger Cut it so that there are two edges along the top, and it connects to the single edge like a V like in the image below. Do this for each finger.

Do this again to create the 2nd set of joints on the fingers, then go to the perspective viewport and select all the verts in the lower half of the finger, and ONE pair of verts from the top joint, and rotate the finger downloads (you'll have to move the verts so they line up correctly after you use rotate).

Do it so it looks like the image below.

Repeat this on the 2nd knuckle, and do it for the other fingers

Next select the edges on the side of the hand that would have the thumb and connect them and pull that edge outwards. Select the face where the thumb would extrude from and do a bevel from it. Connect the edges in it to create the joint.

Use Cut to cut in more detail along the top and bottom of the hand itself. Remember N-Gons are no-nos, so any but funky polys that we created need to be cut up into quads and tris.

Once you're satisfied with the hand, move, rotate, and scale it so that it fits with the rest of the body. Go to the Hierarchy Tab and click the Reset Transform and Scale buttons, then the Affect Pivot Only button and in the axis number entry at the bottom center of the screen, set X to 0.

Now you can apply Symmetry to the hand and it'll appear on both sides of the body.