Pixel and point

TL;DR: Don't use nativeScale and nativeBounds, unless you're doing some very low level stuff

What is point and pixel

From https://developer.apple.com/library/content/documentation/2DDrawing/Conceptual/DrawingPrintingiOS/GraphicsDrawingOverview/GraphicsDrawingOverview.html

In iOS there is a distinction between the coordinates you specify in your drawing code and the pixels of the underlying device

The purpose of using points (and the logical coordinate system) is to provide a consistent size of output that is device independent. For most purposes, the actual size of a point is irrelevant. The goal of points is to provide a relatively consistent scale that you can use in your code to specify the size and position of views and rendered content

On a standard-resolution screen, the scale factor is typically 1.0. On a high-resolution screen, the scale factor is typically 2.0

How about scale and nativeScale

From https://developer.apple.com/documentation/uikit/uiscreen

• var bounds: CGRect: The bounding rectangle of the screen, measured in points.
• var nativeBounds: CGRect: The bounding rectangle of the physical screen, measured in pixels.
• var scale: CGFloat: The natural scale factor associated with the screen.
• var nativeScale: CGFloat: The native scale factor for the physical screen.

The scale factor and display mode

See this for a whole list of devices and their scale factors https://www.paintcodeapp.com/news/ultimate-guide-to-iphone-resolutions

The iPhone 6 and 6+ introduced display mode https://www.cnet.com/how-to/explaining-display-zoom-on-iphone-6-and-6-plus/

You can see that currently the iPhone 6+, 6s+, 7+ phones have scale factor of 2.88 in zoomed mode, and 2.6 in standard mode

You can also see that in zoomed mode, iPhone 6 has the same logical size as the iPhone 5

Simulator vs device

This is to show you the differences in nativeScale in simulators and devices in zoomed mode, hence differences in nativeBounds.

iPhone 6+ simulator

(lldb) po UIScreen.main.scale
3.0

(lldb) po UIScreen.main.bounds
▿ (0.0, 0.0, 414.0, 736.0)
  ▿ origin : (0.0, 0.0)
    - x : 0.0
    - y : 0.0
  ▿ size : (414.0, 736.0)
    - width : 414.0
    - height : 736.0

(lldb) po UIScreen.main.nativeScale
3.0

(lldb) po UIScreen.main.nativeBounds
▿ (0.0, 0.0, 1242.0, 2208.0)
  ▿ origin : (0.0, 0.0)
    - x : 0.0
    - y : 0.0
  ▿ size : (1242.0, 2208.0)
    - width : 1242.0
    - height : 2208.0

iPhone 6+ device

(lldb) po UIScreen.main.scale
3.0

(lldb) po UIScreen.main.bounds
▿ (0.0, 0.0, 375.0, 667.0)
  ▿ origin : (0.0, 0.0)
    - x : 0.0
    - y : 0.0
  ▿ size : (375.0, 667.0)
    - width : 375.0
    - height : 667.0

(lldb) po UIScreen.main.nativeScale
2.88

(lldb) po UIScreen.main.nativeBounds
▿ (0.0, 0.0, 1080.0, 1920.0)
  ▿ origin : (0.0, 0.0)
    - x : 0.0
    - y : 0.0
  ▿ size : (1080.0, 1920.0)
    - width : 1080.0
    - height : 1920.0