I actually didnt understand what u want to know but let me tell u a simple way
1. In shutter priority u set the shutter speed and through the meter of camera , camera decides what aperture to select...if the max aperture is reached then it will increase the ISO
2. In aperture priority u select the aperture and the camera meter decides how much shutter speed can be given ....but now the shutter speed can go down to such that u can not hand hold the shot
in dark places or indoors I usually use shutter priority and set it to 1/30-1/50 so that even if my ISO increases my shot do not get blurred.
now metering u know...spot,center and matrix
spot - it will check the light of a perticular spot only
center - it will check the lightning in the middle of the frame
matrix - it will take the whole frame into consideration and check the average light....this usually make the sky white
hope ur question is answered