To create the quadruped you need to add the pelvis, then the spine.
Once you have the spine added you have the the option to add a “leg back”
This is a dropdown/button.
Clicking adds the “leg back”
Clicking and holding will allow you to select the front legs.
As for fitting the character, the joints follow the general skeletal structure of an actual quadruped.
For the hind legs it looks like there is a joint missing at the top…but the bone you seen in the image is the hip bone…so it is static.
Where as the front legs have an extra joint because they include the shoulder blade.