extSDCard as internal Storage - how does Androied manage app & data location
The adopted SD card is actually mounted at /mnt/expand/$UUID

Instead of forcing the user to pick for EVERY file whether they want it stored in the device's internal storage or the adopted storage, they simply made the internal memory unavailable for storage. That is, /storage/emulated is mounted to point to your adopted SD card and your previous internal storage data that actually lived in /data/media is moved over to its new home on the SD card. This gives the added benefit that there is NO ambiguity where apps are storing user data. It always goes to the adopted SD card. 

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reddit: Let's clear up the confusion regarding storage in Android once and for all, including adoptable storage in Marshmallow.