Elliptical machines mainly work the lower body and, if you use the handles, the upper body too. The primary muscles are the quadriceps, hamstrings, glutes, and calves. Because the motion is low-impact and continuous, the hip flexors also help, and the core works to stabilize your posture. If you push and pull the moving arms, you additionally engage the shoulders, chest, back, biceps, and triceps to a moderate degree.
Stepmill machines focus much more heavily on the lower body, especially the glutes, quadriceps, hamstrings, and calves. They also strongly challenge the hip flexors and core, since climbing stairs repeatedly requires balance, posture control, and lifting your body weight against gravity. The glutes often feel especially worked because step climbing involves a lot of hip extension. The calves assist with each step, while the quads do much of the knee-extension work.
In simple terms: ellipticals give a more balanced, full-body cardio workout with less impact, while stepmills are more lower-body dominant and usually feel harder on the legs and lungs. If the elliptical resistance is high or the incline is increased, it can target the glutes and hamstrings more. On the stepmill, taking deeper steps or standing taller can shift emphasis between the glutes, quads, and calves.