In what order are the stack views (subviews) added when calling the addSubview method?

Views are added above all existing subviews in the same order in which addSubview was called. Thus, the last added view will be the topmost in the hierarchy of the parent view's subviews.

Visually, this means that the most recently added view will overlay previous ones if their frames intersect.

Example of adding views:

// Create a parent view
let parentView = UIView(frame: CGRect(x: 0, y: 0, width: 200, height: 200))
parentView.backgroundColor = .lightGray

// Create the first view
let firstView = UIView(frame: CGRect(x: 20, y: 20, width: 100, height: 100))
firstView.backgroundColor = .red

// Create the second view
let secondView = UIView(frame: CGRect(x: 50, y: 50, width: 100, height: 100))
secondView.backgroundColor = .blue

// Add the first view. It will be the first in subviews
parentView.addSubview(firstView)

// Add the second view. It will be the second in subviews and above the first
parentView.addSubview(secondView)

// Now parentView.subviews will contain [firstView, secondView]

The order in the subviews array of the parent view reflects the Z-axis stacking order. The first element is at the very bottom, and the last is at the very top.