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.


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?
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 &amp; 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? 
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.
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.
Note: this is a partial answer.




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


Correct. RAM is not considered part of internal storage, so it is not part of 8GB storage space.


  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?


In your device, yes, they are two separate partitions. However, the terms shown in your device are confusing. Phone is also part of Internal storage. In my device, the Internal storage is listed as Phone Storage and Phone is listed as Internal SD card.

(Note: the trend nowadays is to have a single userdata partition and a completely logical internal SD card partition. See the related info: Confused by the many locations of the virtual /sdcard/ )

Often, the Internal Storage (which here implies the userdata  partition mounted at /data) is formatted with EXT4 filesystem and Phone (which here implies the Internal SD card) is mounted under /storage/ usually, but not limited to, at /storage/sdcard1. This partition may either be formatted with EXT4 filesystem or  a variant of FAT filesystem. You can check the filesystem type by issuing the command

mount | grep -e /data -e /storage/sdcard


Dummy output


/emmc@usrdata /data ext4 rw,nosuid,nodev,noatime,discard,noauto_da_alloc,data=ordered 0 0
/dev/block/vold/179:8 /storage/sdcard1 vfat rw,dirsync,nosuid,nodev,noexec,relatime,uid=1000,gid=1015,fmask=0702,dmask=0702,allow_utime=0020,codepage=cp437,iocharset=iso8859-1,shortname=mixed,utf8,errors=remount-ro 0 0


Next to /data and /storage/sdcard1 (the mount points) here is the filesystem. 

You can also see the filesystem by tapping the particular partition entry in DiskInfo.


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


As per the info here, an app will always have its private data saved inside the Internal storage (under /data/data) regardless of the app  being installed under Internal or External Storage. 

For finer details, see answers for Installing to SDCard by default and Where in the file system are applications installed?

If the device supports USB Mass Storage for data transfer, once it is activated all the apps installed in the external storage would be unavailable for usage. This is a major difference between the two types. The other one pertains to available space. Internal storage compared to External storage, cannot be replaced or upgraded.


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


Android OS is stored under System partition and is mounted at /system. All the rest of the  partitions  (excluding External storage and tmpfs mount points) are accounted for what you can refer to as firmware.



Now the important stuff. For DiskInfo, 1GB=1024MB (per this answer).

Considering all the screenshots of DiskInfo, my calculation gives me 1803MB, 5.5GB and 512KB. Using 1024 units as a base, the grand total is 1803*1024*1024 + 5.5*1024*1024*1024 + 512*1024 = 7,796,686,848 bytes.

As per Gigabyte wiki article (further confirmation), for storage 1GB=1,000MB and not 1,024MB which is often used only for memory, such as RAM. Therefore, the size of your overall ROM in bytes is 8GB = 8*1000*1000*1000 = 8,000,000,000 bytes.

The difference here is of 203,313,152 bytes or 193.9MiB or 203.3MB.

It is something that I'm unable to figure out even when all the partitions and their sizes are listed here. Perhaps somebody else might!