I have no knowledge of Brazilian Butt Lifts specifically, but here is some related information about how fat works in general, which I hope is a good starting point:

Fat cells don’t die easily. They just shrink. See: news.yale.edu/…/study-new-fat-cells-are-created-q…

When performing skin grafts, fat cells retain the characteristics of the original skin location. For example, here is a paper that shows a soldier who had a skin graft from his stomach to his hand, and later developed a kind of “beer gut” on his hand. Content warning: graphic images of open surgery in related articles section if you scroll down. If you are even a little squeamish, do not scroll down. …lww.com/…/does_transferred_fat_retain_properties…