If you are building the project for the first time, you need to run yarn install.
Also, you need to run yarn install if a new dependency is added to package.json.
yarn build builds bundles in production mode.
yarn serve runs the dev server.
You need to launch backend on 9000 port for the server to operate correctly as specified in webpack.dev.js.
If you make any changes, yarn serve will automatically trigger recompilation.
yarn tslint runs linter.