In Erlang, modules are stored as compiled bytecode files with the extension ".beam". When an Erlang module is compiled, the Erlang compiler (erlc) transforms the source code into bytecode that can be executed by the Erlang virtual machine (BEAM).

The compiled bytecode files (.beam files) contain the compiled code for the module's functions, as well as metadata and other information related to the module. This bytecode format is specific to Erlang and is not directly readable or editable by developers.

The .beam files are typically stored in the file system, either in the same directory as the corresponding Erlang source files or in a specified location. By convention, the .beam files have the same name as the Erlang module they represent, with the ".beam" extension.

When an Erlang program is executed, the Erlang virtual machine (BEAM) loads the necessary .beam files into memory. This allows the functions and code within the modules to be accessed and executed during runtime.

It's important to note that when a module is modified or updated, it needs to be recompiled to generate a new .beam file. The new .beam file can then be loaded by the Erlang virtual machine to incorporate the changes made to the module's code.