Understand the storage of my phone?
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!