MonoX searc system is built upon provider concept (see here
for more info) - you can always build your own custom provider for any type of search. The base namespace is MonoSoftware.MonoX.SearchEngine, and there is a number of providers (for groups, blogs, documents, photos, profiles, file search, indexing service...) in the MonoSoftware.MonoX.SearchEngine.Providers namespace. Providers are registered via web.config. If you are going to write your own provider, it must inherit from SearchProviderBase and implement ISearchEngineProvider interface. More info is available in the API documentation.
When you click on the search button, MonoX calls the Search method in all registered providers, aggregates the results and displays them in the search Web part.