Lost facial fat afterwards - does collagen from Ultherapy only tighten the skin, or can it also add volume?

I had Ultherapy four months ago, and I’ve noticed my skin feels tighter, but I’ve also lost facial fat, which has made my face look hollow. I thought the new collagen production would add some plumpness to my face, but it seems that’s not happening. Does collagen from Ultherapy only tighten the skin, or can it also add volume? Additionally, what does Ultherapy target first—fat or skin layers? If I want to regain facial fat naturally without using fillers, is that possible? Would gaining weight