Collections are registered with the
collections extension accepts an array of collections where the key is the name of the collection and the value a callback function that returns the collection.
Alternatively, you can require a file. The required file would be a collection with a return function.
Kirby::plugin('your/plugin', [ 'collections' => [ 'articles' => require 'some/file.php' ] ]);
Once defined, you can use the collection in your templates using the