After reading this wiki and searching for copper for a couple of days, I wanted to add my experience so that other players can benefit.
So I started digging a mine in a world (V8.0), in the desert biome. I wanted copper, so I dug a tunnel, at layer 41, and I started making my way under the "boreal forest biome". Once I arrived under the biome, I dug a 80 cubes long tunnel (3*3*80 to be precise) and never did I come accross copper.
I couldn't find any, I was kind of going insane, (I have found plenty of granite though) so I started going up. After reaching layer 51, and just about 2 minutes in, I started to find copper.
That makes me wonder about the "20 to 30 layers under the surface". I based my researches on the sea level, that's why I chose layer 41 to begin with, was I wrong ? I think so. I think it means literally 20 to 30 layers underground, no matter how high the mountain.
Thats all folks, have a nice day.