[MM3-users] (Custom) Plugins: Proper file path(s) for adding a handler?

I want to add a custom handler that implements customized thread IDs in the system.  I already have the DB logic worked out, but the documentation for plugins is NOT clear on how to define a handler in a plugin, nor does the example plugin in the documentation on Gitlab have any custom handlers.

I'm looking to add a custom handler so that the private pipeline for threaded-subject-list can use thread-subject (the custom handler) instead of subject-prefix (builtin).

I'm unclear how to add this properly in a plugin that would be read properly by Mailman, though.

Any tips on the filestructure needed for the Python code/plugin package in order to properly be set up and work?

Thomas