From the manual page of
Each line of this file contains 9 fields, separated by colons (“:”), in the following order:
Refer to crypt(3) for details on how this string is interpreted.
Alright, let's have a look at
man 3 crypt then. From the notes section:
[..] So $5$salt$encrypted is an SHA-256 encoded password and $6$salt$encrypted is an SHA-512 encoded one.
"salt" stands for the up to 16 characters following "$id$" in the salt. The encrypted part of the password string is the actual computed password.
So, the salt is stored in the password field of
/etc/shadow itself. For more details, have a look at the manual page of crypt(3).