Building a scalable chat application similar to Facebook Messages involves using a transactional cloud database like YugabyteDB, which offers compatibility with popular APIs such as Cassandra and Redis. The application is designed to handle one-on-one messaging, and the architecture emphasizes data de-normalization and co-location for scalability and efficiency. Key components include users, messages, and threads, with unique identifiers for each entity to facilitate operations. The Cassandra API is utilized for the message-list, ensuring data consistency and efficient retrieval of recent messages, while Redis is used for managing thread-lists and snippets, offering ease in updating and fetching data. YugabyteDB's robust architecture allows developers to leverage these APIs' strengths, ensuring the chat app can manage large datasets efficiently.