When a JSS site loads, it gets the current route's layout data. In integrated mode, this comes by a JS object delivered directly to the
renderView() method (server-side rendering), and in connected mode or during route change, the data is delivered by a HTTP call to the Layout Service.
This data contains the name of the components that you see in the rendering item as part of the layout:
Then the Component Factory (part of the JSS app) maps those component names into a JS component implementation (this example is React; each library does it differently):
import IntegratedPage from '../app/components/IntegratedPage';
const components = new Map();
const componentFactory = (componentName) => components.get(componentName);
export default componentFactory;