You frist need to set up two roles, one for the logged in users, and one for the anonymous users. They should both have access to the same app, but the anonymous user should have unauthenticated access.
Next, you can add two different UI components: one showing the whole text, the other only displaying 200 characters. Then, you can set the visibility of the components individually based on “Current user role” or the built-in App Variable “Is Authenticated”, i.e. the UI component displaying the whole text should only be visible to the authenticated users.