macOS Big Sur - Where is the icon file (.icns) for System Preferences.app?
The System Preferences icon...
In macOS Catalina the System Preferences icon, PrefApp.icns, is located at:
/System/Applications/System Preferences.app/Contents/Resources/PrefApp.icns

However, in macOS Big Sur it is now included in its Assets.car file:
/System/Applications/System Preferences.app/Contents/Resources/Assets.car

In Finder, you can right-click on the application bundle and select Show Package Contents to navigate to these files, or copy them using Terminal.
The image below is a third-party utility called Asset Catalog Tinkerer, to which I have no affiliation. You can use it to view the contents and drag and drop out of it the image you want.