I have always had a firm bottom even when everything else is going to pot. If you tend to gain weight in the hips and thighs, you could have rock hard muscles, but if you have some extra weight on you, the fat will mask the muscle deinition, because the fat layer is over the muscle. I only mention it, because if you are tring to spot reduce, that does not work, there is not such thing. If anything you build the muscle ou concentrate on, make it bigger, and if you still have the fat, you make the area bigger.
I zumba, because I love to dance. I almost never do squats even though the zumba teachers seem obsessed with them. I do sometimes do them ballet style, with feet turned out, which in my opinion are much less stress on the knee. You go straight down, back straight (no stress on the back, improves posture, a regular squat you lean forward, it is competely unnatural to me). All the ballet dancers I know had rock hard defined asses, and they never did a foot forward squat. I don’t do any of the turns the zumba instructor does. I just stay straight forward when they turn.
Water aerobics and I also lap swim.
Walking.
We use our gluts every time we use our legs, in my opinion you don’t have to over focus on butt exercises if you move around a lot.