Simple corregated, galvanized barn siding would be attached with gasketed nails or screws. The holes must be drilled by the installer and would be in the hill not the valley of the corregation.
Pole-barn type pre-finished metal panels are attached with a gasketed screw or ring-shank nail. In this case there is usually no need to drill pilot holes
Some standing seam metal roofs employ seaming machine which runs down between 2 panels and folds their edges together. Other standing seam roofs simply snap together. Generally standing seam roofs are fastened at the seam through the use of a special clip which is fastened to the roof structure. The clip is seamed into the roof panels.
There are also many artful old-world ways of installing metal on a roof. None of which I know enough about to even begin.