What are `storage/emulated`, `mnt/sdcard`, `storage/sdcard`?

Question

There have been questions about the old sdcard turning into sdcard0, which I understand to be part of the multiuser feature.

What I don't understand is the 3 paths in the questions. Some of them look like symlinks to me, since files copied to one place show up in another.

Could anyone systematically explain the relationship between those 3 (and any other storage path)? Given that relationship, where should I store my files (e.g. Firefox's Download location, Aldiko's ebook storage directory, etc.)?

Answer 1

"In the case of devices without true external storage, it's still necessary for Android to provide an emulated external storage in order to remain compatible with older apps. In other words the RAM is physically internal (non-removable) but a section of it is partitioned and the Android file-system APIs treat that partition as being "external" and world-readable."

Taken from answer by @Squonk https://stackoverflow.com/questions/23625104/storage-emulated-legacy-vs-storage-emulated-0-vs-data-data-myapp

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As to where to store your downloads at storage/sdcard. The others are symlinks to that location.

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A symlink is a shortcut, a low-level pointer. A symlink will look like the linked file is actually there, rather than it just being a shortcut. Once you tap on it, it’ll be directed toward the actual file location and run/use data from there.