Confused about Metering and Auto Focus on Nikon Cameras?

I've been looking up stuff how to use my camera and I'm quite confused about metering and autofocus.

I know that matrix mode, takes into consideration of light in the frame of the photo and center-weighted is considering the center of the photo and spot is for a single dot in the center.

I'm confused on how do you use the metering, is it automatic do I have to half press a button for it to activate? Do can I use it in manual, aperture priority mode? Also when the Nikon camera focuses automatically will it meter the light or something along with focusing?
Added (1). Do I have to use it in manual*

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How to use flash on a camera properly?

Here's a picture of my attempt at bohek photography of a dead flower.

It was on 105mm at 1/10s, F/5.8, ISO 400, EV -2.0, Flash EV -1.0
Im new at photography and I was wonder how could you get the background brighter when using flash on a Nikon D90. It seems like the flash makes the object near the camera to appear really bright compared to the background.

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What is the lens for a DSLR camera used for?

Is it used for zooming in or focusing while taking a picture or both? If both, do all types of DLSR lenses focus and zoom? What does the lens on the Nikon D3200 do? (focus or zoom or both?)

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