A Google Closure developer uses Google's powerful set of JavaScript tools known as Closure to build complex, large-scale web applications. They utilize the Closure Library, a broad, modular, and cross-browser JavaScript library, for constructing rich internet applications. They also use the Closure Compiler for optimizing JavaScript, checking syntax, variable references, types, and optimizing performance. Furthermore, they manage dependencies with Closure Templates and Closure Stylesheets, which are used for dynamic UI generation and CSS manipulation, respectively. They work on debugging, testing, and enhancing web application performance, ensuring seamless user experience.