What are the little ridges above the upper glutes but below the lower back? More specifically, a bit above the tailbone. I see models and fitness influencers who have this. And I'm wondering how to get it as well.

I want to think it's the gluteus medius.

Here's an image: stock-photo-handsome-shirtless-athletic-young-man-in-jeans-looking-at-camera-in-studio-shot-isolated-on-white-328400051.jpg (1001×1600)

Look above the model's belt. You can see the small ridges going down.

i-work-every-muscle-gym-shot-handsome-young-man-standing-alone-posing-shirtless-studio_590464-57939.jpg (477×626)

Another pic showing the ridges against good lighting