What happens to fillers as they wear off? Do they turn to scar tissue like Alloderm?

So many famous patients of cosmetic surgery look unnatural and have crepey, unnaturally smooth or sagging skin in the areas I would like a more attractive contour without necessarily using implants--paranasal/nasolabial fold area, midface, and jaw. I'd like to know if that's due to repeated use of fillers and how to avoid that.