First, run the development server:
npm run devOpen http://localhost:3000 with your browser to see the result.
All the main pages are inside of src/app. The page for the URL /test can be found in src/app/test/page.js and its CSS can be found in src/app/test/page.module.css.
The individual components are inside of src/components. The Button component can be found in src/components/Button and its CSS can be found in src/components/Button.module.css.
The shared CSS for the website is in src/app/globals.css in case you want to edit the CSS for headings, etc.