What is my nasolabial fold being caused by? Excess fat or sagging skin? What surgery or lift would even out the area? (Photos)

This can be due to weight loss, genetics, aging, etc. I recommend fillers for the folds themselves but also lifting around the mouth and cheeks to build underlying anatomical support. In our practice we combine sculptra with other fillers "layered" on top and lasers, rf treatments, J Plasma and peels to get the most conservative natural look. Please see a facial sculpting expert. Best, Dr. Emer

Hi, Congratulations on your weight loss! Certainly, it makes sense that losing weight would translate into loss of volume on your face. That being said, you are very young ( or at least look to be!) and using fillers to manage this problem doesn't make a lot of sense to me if you want to hold onto a very natural look long term and also to avoid the economic commitment to fillers over your lifetime (and I do fillers in my practice). 
I think a better choice for you (without seeing you in person up close) is to do some skin rejuvenation treatments on your face that will tighten up the skin reducing the laxity and minimizing the folds. This will also help with some of the texture of the skin to look smoother. Something like microneedling with PRP would be a great choice because it would tighten up under your eyes and the nasolabial folds and along the jawline. It would be simple to maintain the results over time and would give you a more natural look. 3 sessions over 3 months should be plenty with once yearly treatment. You can always add a little filler at the end but surely you would either not need to or you would use less thus saving you money as well as being less of a long term commitment. 
Best of luck
Dr. Jamé

Weight loss, obviously while beneficial can have some negative effects on the face in terms of volume loss.  I see you're nasojugal groove is already lacking volume below your eyes.  You may want to have a consult to discuss options but I do think fillers might be a good option to replace the volume you have lost.  Just a small amount of filler under your eyes and in your cheeks will lift the lower face.  You will most likely still need a small amount of filler in the nasolabial folds as well.  Another option you could consider are the PDO threads or PRP injections to stimulate some collagen production in that area.

Patients who lose a large amount of weight also lose volume in the midface. This results in sagging of the face and evidenced by undereye hollows, nasolabial folds, and jowling. Your photos do not show significant volume loss or nasolabial folds but this maybe due to the angle of the photos and lighting. I would schedule an in person consultation with a local aesthetic physician to determine what could be done to address you cosmetic concerns. Fillers can be injected into the midface/cheeks to address the volume loss and to lift the face. Fillers could also be injected directly into the nasolabial folds themselves. I prefer to use Restylane Lyft or Sculptra in the midface/cheeks and Restylane Refyne/Defyne when treating the laugh lines.