Meaning of tilde in Linux bash (not home directory)

It's a Bash feature called "tilde expansion". It's a function of the shell, not the OS. You'll get different behavior with csh, for example.

To answer your question about where the information comes from: your home directory comes from the variable $HOME (no matter what you store there), while other user's homes are retrieved real-time using getpwent(). This function is usually controlled by NSS; so by default values are pulled out of /etc/passwd, though it can be configured to retrieve the information using any source desired, such as NIS, LDAP or an SQL database.

Tilde expansion is more than home directory lookup. Here's a summary:

~              $HOME~fred          (freds home dir)~+             $PWD       (your current working directory)~-             $OLDPWD    (your previous directory)~1             `dirs +1`~2             `dirs +2`~-1            `dirs -1`

dirs and ~1, ~-1, etc., are used in conjunction with pushd and popd.

Those are the home directories of the users. Try cd ~(your username), for example.

Are they the home directories of users in /etc/passwd? Services like postgres, sendmail, apache, etc., create system users that have home directories just like normal users.