In Linux, user passwords are typically stored in the /etc/shadow file. This file is readable only by the root user and contains password hashes for all of the system's users. The passwords themselves are not stored in this file, but rather a hash of the password, which is a one-way transformation of the password into a fixed-size string of characters that is designed to be difficult to reverse. This means that it is not possible to determine the original password from the hash. When a user logs in to the system, their password is hashed and compared to the hash stored in the /etc/shadow file. If the two hashes match, the login is successful.
It's important to note that the /etc/shadow file should be protected from unauthorized access, as anyone who has access to this file could potentially compromise the security of the system by attempting to crack the password hashes.