Authentication can be handled in many different ways in Linux. Password authentication via /etc/passwd and /etc/shadow is the usual default. There is no default password.
A user is not required to have a password. In a typical setup a user without a password will be unable to authenticate with the use of a password. This is common for system users which are used to run daemons, but are not intended to be used directly by a human.
You can configure Linux to allow login to the desktop automatically, or allow login without a password. Authentication is done via PAM, which is highly configurable. The Arch wiki offers the following PAM configuration for login without a password:
If you want to bypass the password prompt in GDM then simply add the
following line on the first line of /etc/pam.d/gdm-password:
auth sufficient pam_succeed_if.so user ingroup nopasswdlogin
Then, add the group nopasswdlogin to your system. See Groups for
group descriptions and group management commands. Now, add your
user to the nopasswdlogin group and you will only have to click
on your username to login.