A PipeWire module is effectively a PipeWire client in an .so file that shares the PipeWire Context with the loading entity.

Usually modules are loaded when they are listed in the configuration files. For example the default configuration file loads several modules:

context.modules = [
    ...
    # The native communication protocol.
    {   name = libpipewire-module-protocol-native }
 
    # The profile module. Allows application to access profiler
    # and performance data. It provides an interface that is used
    # by pw-top and pw-profiler.
    {   name = libpipewire-module-profiler }
 
    # Allows applications to create metadata objects. It creates
    # a factory for Metadata objects.
    {   name = libpipewire-module-metadata }
 
    # Creates a factory for making devices that run in the
    # context of the PipeWire server.
    {   name = libpipewire-module-spa-device-factory }
    ...
]

And the matching libraries are:

$ ls -1 /usr/lib64/pipewire-0.3/libpipewire-module*
...
/usr/lib64/pipewire-0.3/libpipewire-module-metadata.so
/usr/lib64/pipewire-0.3/libpipewire-module-profiler.so
/usr/lib64/pipewire-0.3/libpipewire-module-protocol-native.so
/usr/lib64/pipewire-0.3/libpipewire-module-spa-device-factory.so
...

A module's entry point is the pipewire__module_init function, see PIPEWIRE_SYMBOL_MODULE_INIT.

int pipewire__module_init(struct pw_impl_module *module, const char *args).`

See the PipeWire Module: Example Sink and PipeWire Module: Example Source modules for a general oveview of how modules look like.

List of known modules: