To get this WordPress REST API simple example to work for you, you need to learn how it works in more detail. Official documentation recommends learning about:
Routes/Endpoints - which are mappings of individual HTTP methods to routes known as "endpoints" - you do it using register_rest_route() function, and here you can find more about Routes and Endpoints.
Requests - WordPress REST API defines
WP_REST_Request class which is used to store and retrieve information for the current request.
WP_REST_Request objects are automatically generated for you whenever you make an HTTP request to a registered route. The data specified in the request will determine what response you get back out of the API. Here can learn more about the WP_REST_Request class.
Responses - are the data you get back from the API. The
WP_REST_Response provides a way to interact with the response data returned by endpoints. In your endpoint definition you name the callback (response) function to serve your interaction.
Here can learn more about the WP_REST_Response class.
Schema - Each endpoint requires and provides slightly different data structures, and those structures are defined in the API Schema. If you want maintainable, discoverable, and easily extensible endpoints it is recommended to use the schema. Here you can learn more about the Schema.
Controller Classes - they bring all elements together in a single place. With a controller class you can manage the registration of routes & endpoints, handle requests, utilize schema, and generate API responses. You have already learned about two controller classes:
WP_REST_Response. Here you can learn more about the Controller Classes
Note: Some of this information is taken from the official Wordpress REST APi Handbook