The Outriders chapter will help you figure out how to mine the materials needed to craft items. With this guide page, you'll know what deposits look like and where to find them.
Most deposits appear in separate arenas during missions. An example of an iron deposit is highlighted in the picture below.
Just walk up to them, press and hold the action button (E by default). Your character will start collecting ore for a few moments. Collect the minerals whenever you have the chance - it's always an additional raw material for upgrading your equipment. You can get up to about 200 pieces of raw material from one such deposit.