Sd card and its directory
Checking your pics again: that device seems to have a weird mounting set up, which I've never seen before on an Android device. /storage/sdcard0 indeed is your internal SD. But as soon as the external card is mounted, some kind of "overlay file system" seems to be initialized, blending it in with the internal card as if it were a single device. You can see that by the fact that, when unmounting, some entries from pic#3 disappear (pic#4; of course the files from the external card) while some remain (those on the internal card).

To find out where the external card is mounted physically, you can use the mount or df command at a terminal prompt: no root required for that, a simple terminal app will do or, alternatively, access via adb shell (see: adb and its tag-wiki).



Edit: After having collected df output with and without the external card mounted, the situation looks a bit more confusing. Here's what's happening in your case:


the internal SD card gets mounted to /storage/sdcard0, as it should be.
when the external card is inserted,


the internal one gets unmounted
the external card gets mounted to /storage/sdcard0
the internal card gets mounted to /storage/sdcard0/internalSD

when the external card is unmounted, things reverse again


So /storage/sdcard0 never shows "empty", but the content "changes". Relevant parts of df output to confirm this:


# df run with the external card mounted:
/storage/sdcard0              1.9G …
/storage/sdcard0/internalSD   1023.7M …
# df run after unmounting the external card:
/storage/sdcard0              1023.7M …


Check with the sizes to confirm. Another proof is the mount output:


# mount with both cards mounted:
/dev/block/vold/179:65 /storage/sdcard0 vfat
/dev/block/vold/179:30 /storage/sdcard0/internalSD vfat
# after unmounting the external card:
/dev/block/vold/179:30 /storage/sdcard0 vfat


Here you clearly see the device (first column) mounted to /storage/sdcard0 changes (which explains why I wanted the mount output in the first place). So no UnionFS: the output also clearly shows both cards using VFAT. Changing contents in the /storage/sdcard0 directory are easily explained by the SD cards being "interchanged".