Hi buzzo, the first thing to know about plumbing metals is that “PLUMBER” was the name given to tradesmen  who used the metal “LEAD” a worker of lead, also because the Latin for lead is “PLUMBUM” with the chemical symbol “PB”.
So lead was used for water pipes and is still used for roof flashings in some cases.
Brass is an alloy of copper and zinc used for taps/valves/compression fittings to name a few.
Copper is a raw material used for water and gas pipes/traps/roofing/guttering to name a few.
Stainless steel is used for water pipes/sink benches/laundry tubs/some pipe fittings.
Chromium is used for the protective coating of tap ware/waste traps/plastic accessories to name a few.
Mercury was used for gas testing by substituting water in manometers to test for higher pressures; mercury is 13.6 times heavier than water.
Aluminum is used for roof flashings/some gas and water fittings/gas regulators to name a few,
 hope this helps,cheers.

hi Buzzo, yes the Romans were guilty of inventing our trade, they actually made drinking cups out of lead and suffered lead poisoning because of it, of course plumbers covered all things lead including lead light glazing. I admire your enthusiasm for the trade and hope you do well but encourage you to branch sideways into something less physical and better rewarded such as trade rep/draughting/building inspector and the reason i say this will become more relevant if you read previous posts concerning the politics of our trades,good luck anyway,cheers

Good on you King Buzzo, your absolutely right, plumbers will always be in demand. Its a good trade to have in the back pocket. Once you start learning all the technical stuff you will say "ahhh so that's why!" Just make sure you KEEP UP THE ENTHUSIASM, that will take you far