Hi Bas,
assets are usually located in:
<project>/assets
This folder is added to your project as a “deployment folder” in the project file. All files in this folder are available on all platforms.
If, for example, you add an image called image.png, you can access it from QML and C++ with the path “assets/image.png”.
If, on the other hand, you need the Android specific assets (for use with native libraries and java code), these are located in:
<project>/android/assets
The android folder is generated when you click the “Create templates” button under:
Projects tab > select any Android build > Expand details of "Build Android APK" > "Create templates" button
Hope this helps!
Cheers,
Chris