Here are some examples of double jointed limb references:
I myself am double jointed and I thought why can’t our characters be double jointed too?
Here are a refernce of what a double jointed lib looks like vs a normal limb:
Weight is easier than limbs they have the bodies already made when they make animations they plug it into animations studio “blender” or Photoshop after effects then convert it to code. They aren’t going to do this they don’t even want to draw different body sizes. I can however say you have a better chance at doing double jointed arms and legs since it would require less work.