Just got my first kit and I'm having trouble finding the difference between the plastic tubes and the metal ones. I know about the shader and liner tubes but what's the difference between the metal and plastic ones?

also I'm having trouble matching the needles to the tubes. If I have a cluster of 3 needles, can I use it as a shader or a liner? What tube do I use for one needle?

Thanks!
Please don't be harsh, I'm not looking to start up a shop, I'm learning. If your not willing to help then please don't answer the question.