Those small dimples above the buttocks are called Venus dimples in women and Apollo dimples in men, or medically, sacral/lumbosacral dimples. They form where the pelvis meets the spine, at the posterior superior iliac spine (PSIS), because the skin attaches directly to underlying ligaments.
Once thought purely genetic, their visibility is influenced by body fat and muscle tone. Lower fat and stronger glutes, lower back, and core make them more noticeable. Exercises like deadlifts, glute bridges, back extensions, planks, and Superman moves help enhance them.