This boilerplate was generated using vitejs instead of Create React App. Most of the tutorials online will probably mention CRA, any setup steps will not be applicable to this project but the React parts should be.
You can add SCSS via index.scss or import them in your .tsx files as CSS Modules. Have a look at src/pages/CatFacts for an example of using CSS modules.
If you are running your API locally, you will need to configure vite to proxy any requests to /api to your server. Check out vite.config.ts, it should have a place for you to put the address of your API server.