So, I've finished the consolidation challange of day 32, things are finally starting to make logical sense to me.

I've finished it with some help of previously written code, and I would like some help/clarification about this specific code.

@objc func addItemAlert() {
        //Setup of alert controller
        let alertController = UIAlertController(title: "Add item", message: "Add the item that you would prefer to add", preferredStyle: .alert)
        alertController.addAction(UIAlertAction(title: "Cancel", style: .destructive))
        alertController.addTextField()

        //Is this a closure?
        let submitAction = UIAlertAction(title: "Submit", style: .default) {
            [weak self, weak alertController] action in
            guard let answer = alertController?.textFields?[0].text else { return }
            self?.submit(answer.lowercased())
        }

        alertController.addAction(submitAction)

        //Presenting controller
        present(alertController, animated: true)
    }

    func submit(_ answer: String) {
        let indexPath = IndexPath(row: 0, section: 0)
        shoppingList.insert(answer, at: 0)
        tableView.insertRows(at: [indexPath], with: .automatic)
    }

Specifically these two blocks of code:

        //Is this a closure?
        let submitAction = UIAlertAction(title: "Submit", style: .default) {
            [weak self, weak alertController] action in
            guard let answer = alertController?.textFields?[0].text else { return }
            self?.submit(answer.lowercased())
        }

        alertController.addAction(submitAction)

        //Presenting controller
        present(alertController, animated: true)
    }

    func submit(_ answer: String) {
        let indexPath = IndexPath(row: 0, section: 0)
        shoppingList.insert(answer, at: 0)
        tableView.insertRows(at: [indexPath], with: .automatic)
    }

How does 'sumbmitAction' work? does it run when submit is being called, or is it the other way around? Some clarification on this code would be appreciated thanks alot.

Yes creating UIAlertAction will create closure which is then called by the alert controller if user taps on the associated button.