In the final post of her 'Diving Into Vue 3' series, Sandra Rodgers demonstrates how to use the Composition API to write reusable code. She builds a single-file component with an image on the left and text on the right that changes size based on the image being resized. The post covers topics such as working with the DOM, using lifecycle methods, template refs, and creating composables for reusability. Rodgers also provides a link to her GitHub repository containing various branches showcasing the progression of the project's refactoring process.