Here is how you can copy to the obb folder:
Install https://play.google.com/store/apps/details?id=com.google.android.apps.nbu.files
You can deny network / sensors and when opening just give it storage scopes.
Then go to its app info, install unknown apps, allow from this source on, and turn on allow access to Android/obb folder.
Now it should work to open files by google and scroll down to internal storage and go to obb folder and you can copy whatever you want to there