If I understand you right you have your camera on your phone and you're composing the picture and when something bright enters the screen the lighting 'appears' to change (probably the whole screen gets darker).
The camera will adjust what it thinks the shutter/aperture/iso should be to get the "proper" lighting. How does it know what the proper lighting is? It thinks the frame should average out to 18% grey (tones, color doesn't matter). This 18% works for "most" pictures, but if you have a predominately white/light image it will appear dark and vice versa for dark images appearing bright (blacks appear grey).
The fix is that once you figure out what the "proper" exposure is (a grey card can help) then you lock in your ISO/shutter speed/aperture and then as you move about and zoom in/out and change the proportions of the screen that are light or dark then it won't change. This is why people shoot in manual (one of the reasons anyway).
Edit: you can see a similar question here Can the camera select wrong exposure just because of color?