If you want to use the iOS built-in system icons when, for example, define an UIBarButtonItem. The parameter barButtonSystemItem does the trick.
...
// We use the built-in system icon "Add" (.add)
UIBarButtonItem(barButtonSystemItem: .add, target: self, action: #selector(myViewController.didTapAddButton))
@objc func didTapAddButton(sender: UIBarButtonItem) {
// do something
}