There is a program that uses containerView in Swift, and I want to change it using it.

.xib file is created as follows.

The part linked by the code was as follows.

class RegisterPopupView: UIView, PopupViewContainable, Nibable {
    enum Const {
        static let height: CGFloat = 400
    @IBOutlet weak var containerView: UIView! {
        didSet {
            containerView.layer.masksToBounds = false

And what is actually displayed is the following.

This view is displayed from below in the pop-up. Why are the .xib files created as above displayed in a single line like this?
In the .xib direction, the view is placed on the left and right on the containerView. On top of that, there are text fields and buttons.

Here, for example, I don't need an input field so I just wanted to use only the NAME and create buttons.

When you do this, only the NAME field is displayed as shown below, and the button disappears.

Why is this?