Building a firm and functional "posterior chain" (your glutes and lower back) helps with posture and athletic performance. You don't need a gym; simple bodyweight exercises can be done at home:
Get on all fours and kick one leg back and up toward the ceiling while keeping your knee bent. This specifically targets the glute muscles.
Typically narrower at the bottom and wider near the hips. Simple Fitness Habits
If you're taking photos and want a more "rounded" look, small adjustments like pushing your hips back or slightly arching your back can change the silhouette.
Focusing on how your body feels and moves is the best way to develop long-term confidence. Healthy habits now will set you up for a strong, active future. 3 Simple Moves to Firm Your Butt and Legs
Avoid products like "fake butts" or heavily padded underwear for daily use, as some users report they can cause hip pain or discomfort after long periods.