OBB files are used in apps that need more than 100 megs to work well. This
is apps like MAPS.ME or games that put map info, media, etc. into the OBB
file. Also, OBB files provide a mechanism to deliver large data blobs that
do not need to be part of the APK. For example, a game's assets do not
need to change often, so they can be shipped as an OBB, then APK updates do
not need to include all those assets for each update.
https://developer.android.com/google/play/expansion-files.html
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