Some questions about storage After weeks of trying to understand Android's storage management I'm left with more questions than answers.

I've already figured out how to force-move apps to external SD card, and to convince newly installed apps to directly install to SD card. But I'm still not sure who's against who. Of course, the fact that "SD card" is confusingly used to describe both the phone's own "hard disk" and external cards that you stick in doesn't help. Questions:


All the dozens tools that promise to force-move apps to SD card, one can assume they all mean "external SD card", correct?
Some parts of every app MUST remain on the internal storage, as far as I understand. Some basic core files. Which means: no matter how intensively you move apps to the external SD card, at some point your internal storage will run out of space. You may have an external 128-GB SD card in, but no tool in the world is going to save you from filling up your 5-GB internal storage sooner or later. Correct?
When Play Store tells me "not enough storage", which storage does it mean? How come I try to install a 50-MB app while having much more than that free in the RAM, the ROM, and the internal storage, and I get refused? (I understand that sometimes erasing cache and RAM solves this, but is it always so?) Is there a Store add-on or something like that that makes it all a bit more transparent, showing which downloaded file exactly doesn't fit in which type of storage?
I've heard that some elaborate games weigh 3 GB (never dealt with one myself). How would I theoretically go about installing such a game? I certainly have no such amount of any type of storage, except for the external SD card. Where will the files even be downloaded to?


Thanks!
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