What is /storage/emulated/0/? Recently, I've figured out that if I delete files from /sdcard/Download it deletes files from /storage/emulated/0/Download. And if I add the files into /sdcard/Download it duplicates them in /storage/emulated/0/Download.

So what is /storage/emulated/0/? For what purposes do we have it in our android file system?
/storage/emulated/0/Download is the actual path to the files.

/sdcard/Download is a symlink to the actual path of /storage/emulated/0/Download

However, the actual files are located in the filesystem in /data/media, which is then mounted to /storage/emulated/0 (and often other mountpoints as well)

A Symlink In computing, a symbolic link is a term for any file that contains a reference to another file or directory in the form of an absolute or relative path and that affects pathname resolution. Symbolic links were already present by 1978 in minicomputer operating systems from DEC and Data General's RDOS.