How disk space is partitioned in an Android phone?
Most Android phones have eMMC (flash memory), partitioned in GUID partition table (GPT) just like a PC's hard disk. There are usually less or more 50 partitions on a latest Android phone. OS uses /system while user uses /data mainly. Internal SD card is /data/media. Other partitions are smaller. 

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If you have a rooted phone, you can use Linux tools (parted, gdisk etc.) to manipulate Android 's partition table. But that's really risky.

To fully access storage space, you need root permissions.

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