If you want a slimer waist you can:
1 Remove fat from your tummy with a diet or with a liposuction
2 Use a waist trainer but you have to keep it very tight for a long time if you want to keep the result once your waist trainer removed (your organs will progressively move inside your body, extreme surgery like rib removing can be performed to make it easier and faster, but full time waist trainer is mandatory after such a surgery)
Brazillian butt lift is an optical illusion, you remove fat in your stomach so it IS slimer and you fans fer the fat to you butt so they are bigger and your waist LOOKS even slimer.
Sit ups and other ab workouts give a flat string stomach and tummy, that can be thin, but not as thin as possible (if the thinest waist is your goal)
I hope it can help