What Is Using Up My Storage/Data on Android Phone? First, I appreciate that there are lots of similar questions to this online, but they are often so specific that I haven't been able to solve this question myself (through online forums/questions/etc).

I have a phone with 8GBs of internal storage, as can be seen from the first picture below. ~3GBs of this is used up by Android OS; ~5GBs remains, of which I am using ~4.5GBs. (This leads to issues with not being able to update, etc, due to lack of space.)



All reasonable so far. However, as the next picture shows, of that 4.5GBs used (separate to the 3GBs for Android OS), Apps uses ~2.1GBs and Cached data only ~33MBs.
Hopefully my question is clear now:


  where is the remaining 2.4GBs?
You can examine storage usage from terminal with du  


in your "about phone" settings, tap "build number" 7x (seven) times to unhide the hidden developer settings, then enable USB-Debugging in developer settings  
on PC download platform_tools and busybox binary for your phones cpu architecture (armv7l), rename file "busybox" and copy it to platform_tools folder (i realized toybox is buggy, du works best with busybox)  
open cmd.exe and navigate with cd command to folder where adb.exe is. connect the phone to PC with USB cable, then type the following commands (each line for its own)  

adb devices
adb push busybox /data/local/tmp/
adb shell
chmod a+x /data/local/tmp/busybox
/data/local/tmp/busybox du -acsxhd1 /storage/emulated/0

repeat the last command on different folder for your needs. the last digit in parameters controls the depth (see full list for usage)


du is a command line utility for disk usage  

CUBOT_KING_KONG:/ $ du --help
BusyBox v1.29.2.YDS (2018-08-01 20:19:15 UTC) multi-call binary.

Usage: du [-aHLdclsxhmk] [FILE]...

Summarize disk space used for each FILE and/or directory

        -a      Show file sizes too
        -L      Follow all symlinks
        -H      Follow symlinks on command line
        -d N    Limit output to directories (and files with -a) of depth &lt; N
        -c      Show grand total
        -l      Count sizes many times if hard linked
        -s      Display only a total for each argument
        -x      Skip directories on different filesystems
        -h      Sizes in human readable format (e.g., 1K 243M 2G)
        -m      Sizes in megabytes
        -k      Sizes in kilobytes (default)


if you don't know your phones cpu architecture you can check the SoC from terminal  

adb shell
head /proc/cpuinfo


the busybox binary will remain at your phones temp folder until you delete it

adb shell
rm /data/local/tmp/busybox