I'm the cutest blueberry

If you've never used sfm or another 3D engine like blender then I can see why your confused. but to explain in short. yes you can do that with that model in sfm. there is a bone in her jaw region that controls that section of the jaw. thats not to say its made to do that specifically tho. usually that bones just rotated to open not moved physically away from the head and stretching it.