What is the difference between the exercise made to bulk up and the one to slim down?

I don’t get how that works. Some exercise to lose weight, and others to bulk up. What is the difference between those exercises? Or is it really what you eat that makes the difference?
I’m currently trying to bulk up in the legs and butt, and slim down in the waist and abs area. How can i make sure that my legs don’t get thinner because of the workout, and that my muscles around my abs and waist don’t get bigger?

When trying to bulk up, use heavier weight and less reps…when trying to tone, use less weight and more reps so that it’s more cardio-like

The Firm Cardio Dance Slim Down