To clarify, by internal storage I'm referring to the folder that's shown to you on a PC when you connect your device.
I can use adb shell pm list -f -s/-3 to get the paths of system apps and user apps respectively. I don't know how I can find the internal storage path though.
From my 5 minutes of googling I've found it could be at /sdcard/sdcard0, though I've yet to see that in any official documentation and I have no clue if that's consistent across phones/android versions.