Explanation
PAM modules are organized and stored as shared object files within the /lib/ directory hierarchy. A shared object file is a file that contains executable code and data that can be loaded into memory and used by one or more programs. This allows PAM modules to be dynamically loaded and unloaded by the PAM library as needed, without requiring recompilation or relinking of the programs that use them. The /lib/ directory hierarchy contains subdirectories for different architectures and operating systems, such as
/lib/x86_64-linux-gnu/ or /lib64/. The PAM modules are usually located in a subdirectory named security, such as /lib/x86_64-linux-gnu/security/ or /lib64/security/. The PAM modules have names that start with pam_ and end with .so, such as pam_unix.so or pam_cracklib.so12.
References:
PAM Modules: A section from the Linux-PAM System Administrators' Guide that explains what PAM modules are, how they are named, and where they are located.
An introduction to Pluggable Authentication Modules (PAM) in Linux: An article from Red Hat that introduces the concept and usage of PAM in Linux, which includes a description of PAM modules and their location.