system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: We know the body “burns” fat when losing weight, but how does it get rid of all the extra nerves, blood vessels, and skin cells it creates when gaining weight?
gpt
The cells that become "extra" will apoptose(die) and otherwise be breakdown. This process especially on the protein level (necessary in skin because of lots of collagen). Is not fast and due to severe epigenetic effects from the obesity may not return to normal easily.  Cosmetic surgery is generally used to help this along for large differences. 

They don't get "burned" per se but your body slowly realizes you don't need them and will devascularize the blood vessels and degrade the collagen/keratin in skin. This process is limited by both time and degree of change which is why you get stretch marks(skin collagen synthesis not keeping up)  when quickly get fat/ muscular and get "flaps or a lack of".