Simple explanation if you are familiar with basic principles of how to use computers:
1. Download and extract game files.
2. Run the .apk and give it the permissions it asks for-- allowing it to create a new folder in your storage.
3. Move the specified files you downloaded into the new folder that was created in step 2.
........
this is made much easier by the developer giving you the proper location for these folders
The reason some games like this one need you to do this is because of APK file size limits. They can't exceed 2 GB. By doing it this way they can give you files with much greater size since the APK doesn't have to store all the data (instead you just move it yourself like in step 3)