Learn Spanish (Latin America): Mi hermana hace ejercicio en el parque.

In this lesson, you are learning how hacer followed by an infinitive means to make someone do something (like me hace correr - makes me run). But here, hace is just the standard main verb meaning does. The phrase hacer ejercicio simply means to do exercise or to exercise.

Why use hace ejercicio instead of a single verb for to exercise?

While the reflexive verb ejercitarse does exist, it sounds a bit formal. In everyday Latin American Spanish, people almost always use the phrase hacer ejercicio to talk about working out.

Could I say hace ejercicios with an s at the end?

Yes, but the meaning shifts. Using the singular ejercicio refers to the general physical activity (like getting some exercise). If you use the plural ejercicios, it sounds like she is doing specific sets of movements, or even schoolwork exercises like math problems.

Would it make sense to say por el parque instead of en el parque?

It would, but it changes the picture. En el parque means she is working out at or inside the park. Por el parque means she is exercising around or through the park, which usually implies moving through the area (like jogging).