To the original poster, Ozempic itself didn't dissolve your backside. What it did was help rein in your eating habits, and the resulting weight loss, both fat and muscle, is what actually made up most of that area shrinking. I'd wager, and I'd put real money on this, that cutting out extra sweets was a factor, and that quick fat loss there is exactly what melted away.
Rebuilding that shape means leaning harder into resistance work targeting the legs and rear, glute bridges, thigh work, quad training, that category of exercise, or looking at fat grafting or implants as an alternative. My vote goes to the strength-training route, you're already at the gym after all, right? Booking a session or two with a trainer could get you pointed in the right direction.
Unfortunately putting muscle back on will never move as fast as the fat and muscle disappeared while you were losing. Stair climbing, squat variations, lunges, and other leg-focused movements on a regular basis will help rebuild the actual muscle shape underneath, rather than leaving you with just a thin fat layer over very little tone.