Hi.

There is the claims of what is better or not; the copper, the iridium, the iron, the platinum ones...

Are the expensive metals used in tiny amounts only on the spark eroding prone tips or is there a larger component using such ?  I would suspect the industry would just save the $ and barely coat/plate only the critical surfaces.  What do you know ?

Not something I've ever looked into to be honest, but given that manufacturers say they should last 60,000 miles before changing I'd imagine they have done their calculations reasonably accurate, and I'd like to say I don't see spark plugs failing in service nowadays like I used to 20 years ago.  

The electrodes go through ceramic with a very strong interface. Not every metal achieves that. I suspect refractory, corrosion resistant metal covers only the tip in the combustion chamber.