How do people take pictures like this?

Whereas the subject has the flash and the background doesn't. Like the background is dark?

Or flash even being used?

I have the nikon d3000

Fast shutter speeds, like 1/250

Directional light. Harsh strobe light on the top left of the model.

Sometimes the greatest questions have the simplest answers!

it was taken outdoors at night (probably in a large open space like a field)
needless to say a flash was used on the subject, from the looks of it a built-in one. I would say its a p&s camera.

Flash close to subject & subject distant from any background. Either outside at night or indoors with the camera at max sync speed (1/200 or thereabouts) to kill the ambient light.

That shot was likely on camera flash due to the shadow direction & hard edge (small light source).

If you keep your hand in front of your camera and take picture with flash other background would be darker than you.

in your picture the back group is dark already as it is on outside