What exercises should I do to get a body like her? She's slim and athletic but her muscles aren't too defined. Should I do heavy weights, circuit training, long/low-impact cardio?

Long low impact cardio. Is a must start with fast walking. Then progress to burpies 10 down at first. As for weight training stick with low weight with a lot of reps.

Cardio training get skinny and slim then do some squats. Lift lightly.

She looks like a long distance runner to me. She also looks like she is naturally petite.

Google" Kia Kaha", she looks a lot like your model. Kia is a Malaysian marathoner.

you should do a squat and run as far as you can everyday