This is a very helpful dictionary especially for learners of English: https://www.oxfordlearnersdictionaries.com/definition/english/sheet?q=sheet flat thin piece ​a flat thin piece of any material, normally square or rectangular sheet of something e.g. a sheet of glass/plastic/plywood/steel sheet metal (= metal that has been made into thin sheets)

So sheet metal is flat. But corrugated means the flat sheet is pressed to form waves in the metal. Google corrugated and look at images. It is difficult to describe in chat. lol Have a good day.

sheet means metal that is rolled out flat. Like a SHEET of paper.

Sheet metal is rolled thin. Like a bread dough.