A Bootstrap developer designs responsive websites and web applications using Bootstrap framework. They ensure optimal user experience by creating a visually appealing interface that adjusts to various screen sizes. They utilize HTML, CSS, and JavaScript, which are integral to Bootstrap, to build interactive elements and implement functionalities. They often work closely with back-end developers to integrate server-side functionalities. Additionally, they debug and fix issues, optimize web performance, and maintain the quality and efficiency of the website. Their role may also involve updating to newer versions of Bootstrap, ensuring compatibility with different browsers, and following SEO best practices.