Passwords stored and how are they encrypted in OS X

You can have users without passwords. It's just a very frowned upon practice.
The low level tools like passwd support it (passwd -d to make a password empty). But the high level friendly tools that are provided by the distribution are generally what don't allow it.

On Ubuntu/Debian mkpasswd is part of the package whois and implemented in mkpasswd.c which as actually just a sophisticated wrapper around the crypt() function in glibc declared in unistd.h. crypt() takes two arguments password and salt. Password is "test" in this case, salt is prepended by "$6$" for the SHA-512 hash (see SHA-crypt) so "$6$Zem197T4" is passed to crypt().

Maybe you noticed the -R option of mkpasswd which determines the number of rounds. In the document you'll find a default of 5000 rounds. This is the first hint why the result would never be equal to the simple concatenation of salt and password, it's not hashed only once. Actually if you pass -R 5000 you get the same result. In this case "$6$rounds=5000$Zem197T4" is passed to crypt() and the implementation in glibc (which is the libc of Debian/Ubuntu) extracts the method and number of rounds from this.

What happens inside crypt() is more complicated than just computing a single hash and the result is base64 encoded in the end. That's why the result you showed contains all kinds of characters after the last '$' and not only [0-9a-f] as in the typical hex string of a SHA-512 hash. The algorithm is described in detail in the already mentioned SHA-Crypt document.