If you are using an IPhone then if you tap the screen a yellow box will appear, this shows the focus point. So if you tap else where you can change where the phone focuses. To keep the focus there, tap and hold in the yellow box and ae/ lock will appear. This holds the focus for that shot.
You can turn the phone flash off which will stop the reflections at the top of the screen, it looks like a lightning flash.
Next to the yellow box is a sun symbol. This allows you to adjust the exposure, to make the image lighter or darker ( same as as changing the time the shutter is open) Potential problem in the dark is camera shake, as your body has a tremor so long exposure will make the shot blurred.
Essentially you want short exposure / high speed to catch fast action and Long exposure for slow moving shots but it’s possible to be blurred.
The exposure time affects how much light is used. This is controlled by what used to be the aperture, the f stop. I don’t think this or iso can be changed in the basic camera app but you can obtain similar results using a third party app. It’s sometimes called a depth of field setting. The depth is how much is in focus in the picture. A short depth, say for macro, would show a leaf but the background is blurred. A long depth, most phones standard setting, gets most things in focus but can produce boring pictures. One way around this, other than a third party app, would be to switch to portrait mode. There are probably some pro photographers on here with some better advice, maybe do a search ;-) But try turning the flash off and playing with the focus point , ae lock. If you’ve a different smart phone I’d assume any that compete with Apple will have similar features.