Standard hash function of Linux So I went through my /etc/shadow file in Linux and discovered that my passwords were stored in SHA-512-based crypt ('sha512crypt') mode, so SHA-512. But why? I mean SHA-512 is not even a secure password hashing function. Why not bcrypt or Argon2 or scrypt ....
And how much iteration does this hash even have? 

PS: Does $2$ "Blowfish" mean bcrypt ? And if yes, then why do they use the name of the block cipher?
Though not my normal recommendation, the SHA-512-based password hash as implemented in Linux is a fine password hashing algorithm.  It is based on a well-understood cryptographically secure hash (SHA-512). It has a significant size salt and configurable number of rounds (default 5000). I would not feel insecure with such a hashing algorithm.

$2$ is indeed bcrypt, but note there are several variants around sometimes marked 2a or 2b and sometimes not marked. So, though bcrypt is considered very secure if you need a standard password hashing function, you need to be careful when choosing bcrypt.