Understand the storage of my phone?

(I just updated my post to clarify my questions. Hope that can help to make myself clearer.)

My phone is Kyocera Hydro Icon with Android 4.3.

1. 
   

   The website where I bought the phone says

   
   

   
   

   Memory

   
   

   Internal: 8GB, 1.5GB RAM

   
   
   

   
   

   The 1.5GB RAM isn't part of the 8GB "internal", correct?

   
   


2. 
   

   On my phone, I go to Settings -> Device -> Storage which says:

   
   

   

   
   

   Are "internal storage" and "phone" two partitions (and thus two file systems)? If yes, what are their mounting points (directories), or how can I find out their mounting points?

   
   

   Both "internal storage" and "phone" store "Apps (app data & media ...)". What are the differences between the apps stored on the two places?

   
   

   Where are the firmware and OS (Android) stored in my phone, "internal storage", "phone", or somewhere else?

   
   

   The total space of "internal storage" is 4.42GB, and the total space of "phone" is 1GB. But the website says I have 8GB "internal". Is it correct that there are still 8 - 4.42 - 1 = 2.58GB left? Has the remaining 2.58GB been used to store some files? If yes, what files are stored there, or how can I find it out? Is the remaining 2.58GB a partition (and thus a file system)? What is or how can I find out its mounting point?

   
   
   


3. 
   

   Output of Diskinfo (under the expert setting, unmounted and other partitions checked)

   
   

   

   
   

   The sum of the listed partitions' sizes is (800+643)MB + 4.5GB + 1GB = 1.41GB + 4.5GB + 1GB = 6.91GB, which is still smaller than 8GB "internal" given by the shopping website.

   
   


4. 
   

   Output of df

   
   

    $ df                                                         
   Filesystem               Size     Used     Free   Blksize
   
   /dev                   672.6M   128.0K   672.5M   4096
   /sys/fs/cgroup         672.6M    12.0K   672.6M   4096
   /mnt/asec              672.6M     0.0K   672.6M   4096
   /mnt/obb               672.6M     0.0K   672.6M   4096
   /system                786.5M   768.0M    18.5M   4096
   /data                    4.4G     3.6G   802.9M   4096
   /cache                 632.1M    25.7M   606.4M   4096
   /persist                 7.8M     4.8M     3.0M   4096
   /firmware               64.0M    56.9M     7.1M   16384
   /sysprop                 7.8M     6.0M     1.9M   4096
   
   /carrier                19.6M     4.0M    15.6M   4096
   /mnt/media_rw/sdcard0: Permission denied
   /mnt/secure/asec: Permission denied
   /storage/sdcard0         1.9G   170.4M     1.7G   32768
   /mnt/media_rw/sdcard1: Permission denied
   /storage/sdcard1      1023.7M   825.6M   198.1M   16384
   

   
   

   The sum of the file systems' sizes is (672.6*4 + 786.5 + 632.1 + 7.8*2 + 64 + 19.6 + 1023.7)MB + 4.4GB = 5.11GB + 4.4GB= 9.51GB (note I convert from a MB value to a GB value by dividing MB value by 1024. I also didn't add /storage/sdcard0 into the sum , which is for the external SD card). It is greater than 8GB "internal" shown by the shopping website. Why is that?

   
   

   The sizes of /system, /cache and /data given by df are 786.5M, 632.1M and 4.4G, while the sizes of System, Cache and Data (userdata) given by Diskinfo are 800MB, 643MB and 4.5GB. why do they not match? (The size of /storage/sdcard1 given by df and the size of SD card (internalstorage) given by Diskinfo match. )

   
   

Thanks.