The ability to shift form is entirely based in the soul and does not require the use of channeling mana nor does it make the person a mage. Effects that normally stop the use of magic cannot stop a shifter from changing form. Shifter souls are chimerical, originating from the eleventh realm, although it has been long-forgotten to ancient history how chimeras wound up in the Primal and Planar Realms to begin with.
Shifters are not necessarily simply a humanoid who can turn into an animal. If a shifter mates with an animal of the appropriate species while in animal form, the offspring can be an animal that can turn into a humanoid.
When a shifter mates with a non-shifter, the children tend to favor the physical appearance of the non-shifter parent. For instance, a human-appearing werewolf who mates with an elf will have children that look entirely elven rather than half-elven. However, most of the offspring of shifters and non-shifters on Lezaria will be non-shifters, while shifter offspring are more common on the planes. If two shifters of different types interbreed, the offspring might have either or both of the parents' forms. For instance, the children of a werewolf and a kitsune could be either a werewolf, a kitsune, or be both and able to turn into either a wolf or a fox. Shifters with multiple forms are very rare on Lezaria as their territories don't really overlap, while they're much more common on some planes.