Android is written to consider the SD card as storage external to the system, and defaults to the system as storage. (For a number of reasons, one of which is that some code just won't run from the SD card, another is that the eMMC storage - the internal storage - can take a lot more writing than an SD card can, so the SD card will last a lot longer if it's used as write-once-read-many - like pictures or music. And many camera apps can be set to use the SD card by default. [Since most of the music you'll have is downloaded, and since much downloading is of apps, downloading defaults to the system.])