So you know which wire is which. Your question is a bit like asking "Why does each house in a street have a different number or house name?" In any household electrical appliance there is always at...
The outer jacket color can be any color.the inner 4 twisted pairs are blue and white striped and solid blue, orange and white striped and solid orange, green and white striped and solid green, brown...
There isn't a specific diameter. The thicker the wire the more current it can convey. This diameter will be part of your calculations for your circuit.
There are a number of ways copper wire can be insulated - or has been insulated in the past - but today, the most common insulating material for copper wire (by no means the only one, though), is...