Some questions about storage
Many questions in one, but …


No. They all mean whatever the system sees as "primary external storage". If you've got an "internal SD card" and an "external SD card", this usually means the internal.
Nes. Depends on. Without root powers, this is correct. With root powers there are ways around – see e.g. our link2sd tag-wiki (for a broader intro, also see our app2sd tag-wiki)
the devices internal storage (i.e. /data/data)
if you don't have that much internal storage left, you don't deal with them at all. Initial download goes to internal storage, so no way1.




1: note that with Android 6+ you can adopt the SD card as internal storage to work around this limitation, on the cost of other limitations