Where is the light sensor located in DSLR Cameras?

The reason why I ask, is because I am buying a entry level DSLR for beginners. I'm new to ISO, Aperture, and Shutter Speed. I'm buying a Nikon D3200 which I already put on lay-a-way. I know that the Nikon D3200 works great for taking pictures in daylight or really well lit rooms. But I am wanting to take night shots as well. I honestly don't know where the light sensor is located in DSLR's, I've been told that the light sensor is in the lens. I wanted to know if I could just buy a better lens for taking photos at night or just in darker situations, or is the sensor in fact inside the camera itself? Because I don't want any Noise/Grain in the picture.

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