What is a good way to think of these layout contraints - swift

Question

I am learning about layout constraints and find it a bit confusing why the last line of NSLayout Constraints for the trailing anchor mentions a view instead of loginView? Is there any good logical way to think of this? Struggling to imagine what is written.
let loginView = LoginView()
view.addSubview(loginView)
NSLayoutConstraint.activate([
loginView.centerYAnchor.constraint(equalTo: view.centerYAnchor),
loginView.leadingAnchor.constraint(equalToSystemSpacingAfter: view.leadingAnchor, multiplier: 1),
view.trailingAnchor.constraint(equalToSystemSpacingAfter: loginView.trailingAnchor, multiplier: 1)
])

Answer 1

To clarify the "flipping" between:
loginView.leadingAnchor.constraint(...)
and:
view.trailingAnchor.constraint(...)
Both of these sets of constraints will give the same result:
NSLayoutConstraint.activate([
loginView.heightAnchor.constraint(equalToConstant: 120.0),
loginView.centerYAnchor.constraint(equalTo: view.centerYAnchor),
loginView.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 8.0),
loginView.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: -8.0),
])
NSLayoutConstraint.activate([
loginView.heightAnchor.constraint(equalToConstant: 120.0),
loginView.centerYAnchor.constraint(equalTo: view.centerYAnchor),
loginView.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 8.0),
view.trailingAnchor.constraint(equalTo: loginView.trailingAnchor, constant: 8.0),
])
In each case, we're telling auto-layout to put the trailing-edge of loginView 8-points from the trailing-edge of view.
Which approach to use really comes down to individual preference: Do I like using all Positive values, with order-flipping? Or do I like using Positive values for "left-side" constraints and Negative values for "right-side" constraints without order-flipping (obviously, flip the terminology for LTR locales).
Starting with iOS 11, Apple added the concept of system spacing - which changes based on device size, accessibility options, etc - which we can use instead of hard-coded values.
We have equalToSystemSpacingAfter (and equalToSystemSpacingBelow), but we do not have equalToSystemSpacingBefore (or equalToSystemSpacingAbove).
So, if we want to use system spacing, we must "flip" the constraint order:
NSLayoutConstraint.activate([
loginView.centerYAnchor.constraint(equalTo: view.centerYAnchor),
loginView.heightAnchor.constraint(equalToConstant: 120.0),
loginView.leadingAnchor.constraint(equalToSystemSpacingAfter: view.leadingAnchor, multiplier: 1),
view.trailingAnchor.constraint(equalToSystemSpacingAfter: loginView.trailingAnchor, multiplier: 1),
])

Answer 2

The code you posted is defining a set of layout constraints for the loginView object. The constraints specify how the loginView should be positioned within its parent view.
In the last line of the code, the view.trailingAnchor is being used as the reference for the trailing edge of the loginView. This means that the loginView will be positioned such that its trailing edge is aligned with the trailing edge of the parent view.
In general, when working with layout constraints, it is important to think about the relationship between the views being constrained and the constraints themselves. In this case, the loginView is the view being constrained, and the constraints are defining how the loginView should be positioned relative to its parent view.

Answer 3

view means self.view. This is a UIViewController; it has a view. This is the view that will contain the loginView; you can actually see the loginView being added to the view controller's view as a subview, right there in the code.
So this code inserts the loginView into self.view and then proceeds to describe the physical relationship between their sizes and positions.