I've done some research and testing on my own and found the correct answer:
The Start screen patterns/wallpapers are not stored in the location shared in previous answers. They are stored as resources inside one .dll file:
For each pattern, there are five versions:
- The mini preview of the pattern displayed in PC Settings.
- Background image for devices with low resolutions.
- Background image for devices with medium-resolutions.
- Background image for devices with high resolution.
- Large preview of how the Start screen will look when using the selected pattern, shown in PC Settings.
To extract these images, you need to use a special tool named Resource Hacker.
Regarding the copyright - what has been shared previously is correct. You can't use them in your app even if its a free app.
Complete information, including a step by step guide on how to extract these patterns from the imageres.dll file, can be found here: Where are the Start Screen Patterns Located & How to Extract Them?.