Sheet metal working is taking thin pieces of steal and bending them to size and shape. Air ducts can be all shapes and sizes, typically air ducts are round with a 1-foot diameter. Duct work is held up by bands and is fastened to one another by either tacking or welding then wrapped with duct tape (no not duck tape).
To go out and start working on duct work you need to know what you’re doing. Most people get an apprenticeship and get into the trade that way but you can also go through the union and they would learn the trade through them.
Why I want to be a sheet metal worker