I'll try to explain to the best of my knowledge.
The phone has 16GiB of flash memory used for storage. Note that this is GiB ie not real Gigabytes but the sort disk manufacturers use in adverts. Consequently the amount of internal memory is about 14.8GB (real GB). Some of this space is reserved for the OS itself. That leaves about 12GB of free space.
If you go to Settings->Storage the top category titled "SD card" is your removable micro SD card. On my phone this shows 29.88GB free on my 32GiB micro SD card. This space is not normally used by the OS at all so you can do what you like with it. This is mounted on /sdcard/external_sd.
The category below, "USB storage" is the internal memory available for user use. This is mounted on /sdcard.
Without rooting your phone, you won't have access to the "Device memory" and in any case, you don't want to start messing around with that or you could screw up your phone if you don't know what you're doing.
On my phone the external SD card is almost empty but it looks like yours is almost full. How large is it and what's on it? It looks like you need to check to see what is filling up your external SD card. Have a look what's there and depending what it is, you can probably just move it from /sdcard/external_sd to somewhere in /sdcard using the file manager.