A BEM CSS developer utilizes the Block, Element, Modifier methodology for creating scalable and reusable front-end code. They structure CSS classes to make them more readable and understandable, improving team collaboration. They design blocks as standalone entities, elements as parts of blocks, and modifiers as variations of those elements. This ensures a consistent structure, reducing redundancy and increasing efficiency. They also manage the interactions between these components, ensuring the overall visual coherence of the website or application. Their work contributes to cleaner code, easier maintenance, and better performance in web development.