Core is the data for rn-matrix - you can use Core on it's own if you have no need for built in UI components. However, if you only use Core, I recommend referencing the UI package as an example of implementation.
The UI package has default UI components like an inbox, message list, and composer which simplify the process of adding chat to an app. These are customizable and it's the recommended method of using rn-matrix.
Find installation, usage examples, and more at https://rnmatrix.com