Building a weather app using JavaScript (Node.js) and the OpenWeatherMap API involves leveraging a pre-defined interface between clients and services, which allows for platform-independent access to vast meteorological data sources. The process begins with obtaining a RapidAPI account and an API key for OpenWeatherMap, enabling the testing of endpoints that provide weather data based on parameters such as city names or coordinates. The tutorial guides users through setting up a Node.js project with necessary modules like Express, Unirest, and EJS, which facilitate server management, API requests, and dynamic HTML content rendering, respectively. The example project demonstrates fetching weather data for a specific location, such as the Colosseum in Rome, and rendering the results in a dynamic HTML format using EJS templates. The tutorial emphasizes customizing the presentation by creating an HTML template that displays data such as temperature, humidity, and weather conditions in a structured table format, enhanced with CSS for aesthetic appeal.